Billing is where money is recorded. Every charge is a line on an invoice; every payment, refund and deposit is a transaction; every client has a credit balance in their own currency. Invoices come from orders, from the daily renewal run, from billable items, from accepted quotes, from debit notes and from staff writing them by hand. This chapter covers the Billing menu (Transactions List, Invoices, Billable Items, Quotes, Credit and Debit Notes), the client credit page, Settings > Tax Configuration, the numbering and invoice settings, how renewal dates work, and in brief the automation and the client's view. Payment gateways (online payment, saved cards, the Gateway Log) have their own chapter; the catalogue prices are in Catalogue and the order flow in Orders.
Invoices
Statuses
| Status | Meaning |
|---|---|
| Draft | Written by staff but not published; clients do not see it, no email goes out, it has no number yet. |
| Unpaid | Waiting for money. An unpaid invoice past its due date is shown as Overdue (a view, not a separate status). |
| Payment pending | A payment is on its way (for example started at an online gateway and not confirmed). Treated as open. |
| Collections | Still open; a label for invoices you are chasing. Treated as open. |
| Paid | The balance is covered by payments and credit. |
| Cancelled | Closed without payment. |
| Refunded | Was paid, and every payment on it has been refunded. |
"Open" below means unpaid, payment pending or collections.
Billing > Invoices
The Invoices list (permission invoices.list) has sub-entries in the menu for Paid, Draft, Unpaid, Overdue, Cancelled, Refunded, Collections and Payment Pending. Filters: number or client (name, company, email); Status, including Overdue; Issued from and To; Due from and Due to; Payment method; Total from and Total to. A link from a client's page adds a client filter. Columns: number, client, issued, due (red when overdue), total, balance for open invoices, payment method, status. Under the table, the total and count of the listed invoices per currency.
Buttons: Create Invoice (invoices.create) and Generate due invoices (invoices.generate_due), which runs the renewal invoicing now, for all clients or, when the list is filtered to a client, for that client only (see "Renewal dates" below).
Bulk actions on ticked invoices (invoices.manage; Cancel needs invoices.cancel, Delete invoices.delete):
| Action | What it does |
|---|---|
| Mark paid | Records a payment for the balance with the invoice's payment method (or "other") and marks it paid; open invoices only. |
| Mark unpaid | Sets cancelled, draft, payment pending or collections invoices back to unpaid. |
| Send reminder | Emails the payment reminder, or the first overdue notice when the due date has passed; open invoices only. |
| Merge | Joins the ticked invoices into one (see Merge below) and opens it. |
| Cancel | Cancels draft and open invoices. |
| Delete | Deletes invoices that can be deleted (see Delete below). |
Invoices not in a state for the action are skipped and counted in the message.
Creating an invoice by hand
Billing > Invoices > Create Invoice (invoices.create):
| Field | What it does |
|---|---|
| Client number | The invoice is in the client's currency. |
| Due date | Defaults to seven days from today. |
| Payment method | A gateway or manual method, or "Not chosen"; empty uses the client's default method. |
| Lines | Description, amount and Taxed for each line; five empty rows (leave unused ones empty). Taxed is ticked by default when "Tax lines added by hand" is on. |
| Notes shown on the invoice | Printed on the invoice. |
| Save as a draft | The client does not see it until you publish it. |
| Email the invoice to the client | Sends invoice_created (not for drafts). |
Tax is worked out from the client's country and state and your tax rules at the moment the invoice is created, and the tax names and rates are stored on the invoice. With "Keep the client's details as they were when the invoice was made" on, the client's address as of that day is stored too.
The invoice page
Billing > Invoices > an invoice (invoices.view). Across the top: Print (a printable page), PDF (download), Credit note (opens a new credit note for this invoice; notes.create), Publish for drafts (with "Email it"; invoices.manage), Cancel invoice (invoices.cancel; open and draft invoices) and Delete (invoices.delete; disabled with the reason when it cannot be deleted).
Three cards:
- Invoice: client, issued and due dates, paid date, payment method, and the order it came from.
- Balance: total, credit used, paid, balance, and the client's credit with a link to the credit log. With
credit.manage: "Apply credit" with an amount (open invoices, when the client has credit and something is owed) and "Take credit back" (invoices not paid that have credit on them). - Add payment (
invoices.manage): date, amount (prefilled with the balance), method, reference, fees, and "Send the payment confirmation". Payments can be added to open and to paid invoices; money beyond the balance becomes the client's credit. With a kept payment method (a saved card) and a balance, "Charge now" charges it through the gateway.
Lines: on draft and open invoices, staff with invoices.manage edit the lines in place: description, amount, Taxed, Remove, and two empty rows for new lines (kind "item"), then Save lines. The totals (subtotal, each tax, credit, total) are worked out again from the lines. On other invoices the lines are shown read-only; a star marks taxed lines.
Transactions: every payment and refund on the invoice with date, method, reference, in, fees, out. On each payment not yet fully refunded: Reverse (invoices.manage) and Refund (invoices.refund), described under Payments.
Details (invoices.manage): issued and due dates, payment method, notes shown on the invoice, and, while the invoice can still be changed (see Split), the tax by hand: a tax name and rate and a second tax name and rate. Changing them recounts the invoice with those rates on the lines ticked Taxed. Tax normally follows the tax rules; change it here only for this invoice.
Email and status (invoices.manage): send the invoice email, the payment reminder or the first, second or third overdue notice now; and set the status to Unpaid, Payment pending, Collections or Draft. Setting a paid invoice back to unpaid does not undo what paying it did; use refunds for money returned.
Numbering
Every invoice has an internal id, which is also its number until a sequential number is given. The settings are on Settings > General Settings, tab Invoices:
| Setting | What it does |
|---|---|
| Number invoices in sequence | On: invoices get a number from the counter. Off: the id is the number. |
| Give invoices their number | When paid (unpaid invoices show their id until then) or When created. A draft gets its number when it is paid; publishing it does not number it. |
| Invoice number format | {NUMBER}, {YEAR}, {MONTH}, {DAY}, for example INV-{YEAR}-{NUMBER}. |
| Next invoice number | The counter's next value. |
| Start numbering again | Never, Every month or Every year: the counter goes back to 1 when the month or year changes. |
| Invoice id increment | Ids step by this much (1 to 999). |
| Next invoice id at least | The next id is at least this, to carry on from a previous system; 0 means no change. |
| Set the invoice date to the payment date | When paid, the issued date becomes the payment date. |
| Call unpaid invoices pro forma | The printed invoice says pro forma until it is paid. |
Other settings on the same tab: Attach PDF invoices to emails and PDF paper size (A4 or Letter); Keep the client's details as they were when the invoice was made; Clients may pay several invoices at once (mass pay); Clients may change the payment method of an invoice; Group identical invoice lines; Cancel unpaid invoices of cancelled services; Late fee type, Late fee and Smallest late fee (below); Invoice the next period even if the last one is unpaid (under Renewal dates).
Who the invoice is addressed to
An invoice is addressed to the account holder, or to the account's billing contact when one is set on the client's profile. A billing contact without an address uses the account's address; its company and tax number fall back to the account's. Invoice emails go to the billing contact as well. With "Keep the client's details as they were when the invoice was made" on, the addressee is stored on the invoice and later changes to the profile do not alter old invoices; otherwise the printed invoice shows the details as they are now.
Credit on invoices
- With "Use credit on new invoices automatically" (General Settings, Credit) on, a client's credit is applied to each new invoice (orders, renewals, billable items, quotes, debit notes). An invoice covered in full is paid at once with method "Account credit".
- Staff apply or take back credit from the invoice page; clients apply their credit from their own invoice page.
- Credit never pays an add-funds invoice.
- Credit used on an invoice counts towards its balance like a payment, but is not a transaction.
Split
"Move ticked lines to a new invoice" appears under the lines while the invoice is draft or open, has no payments and no credit on it. Tick "Split off" on the lines to move (at least one must stay). The new invoice is for the same client with the same status, dates, payment method and tax, with the note "Split from invoice #N", and both are recounted.
Merge
Tick two or more invoices in the list and press Merge. All must be draft or open with no payments and no credit, for the same client and currency. Their lines move to the oldest of them; the others are cancelled, empty, with the note "Merged into invoice #N"; orders that pointed at a merged invoice point at the joined one; the due date becomes the earliest of them.
Cancel and delete
Cancel closes a draft or open invoice without payment. Paying a cancelled invoice is refused; set it back to unpaid first. Cancelling an order cancels its unpaid invoice (see Orders), and the daily run cancels invoices unpaid too long when that is set (below).
Delete removes the invoice and its lines and detaches it from its order. It is refused when the invoice has payments recorded ("Cancel it instead"), when credit was used on it ("Take the credit back first"), or when it is paid with a total above zero.
Print and PDF
Print opens the invoice as a page to print; PDF downloads it as Invoice-NUMBER.pdf. The PDF has a coloured band at the top (PDF accent colour, General Settings > Invoices), your logo (Logo address: a PNG, JPEG or GIF on this site or elsewhere, fitted into 180 × 60 points; an SVG or unreadable logo shows the company name instead), the invoice number with a coloured status label (Paid green, Unpaid amber, Overdue and Collections red, Refunded purple, Cancelled and Draft grey), a strip with the invoice date, due date, paid date, payment method and amount due, Invoiced to (with the contact as Attn) beside Pay to, the lines, taxes, credit applied, the balance in a highlighted box, a Transactions table (date, payment method, transaction ID, amount) and the notes. Every page ends with your company name and website, "PDF generated on" with the date and time in the site's time zone, and "Page X of Y"; long invoices continue on new pages with the table header repeated. Labels follow the client's language when the PDF font can show it (Western European languages); others, such as Urdu, stay English in the PDF. A logo on another site is downloaded once a day and kept in storage/cache. With "Attach PDF invoices to emails" on, the PDF is attached to the invoice, reminder, overdue and payment received emails. Reports > PDF batch can print many invoices in one PDF.
Invoice emails and reminders
| Template | When |
|---|---|
invoice_created (New invoice) | When an invoice is made for an order, by the renewal run, from billable items, a quote, a debit note or by hand with the box ticked, and when a draft is published with "Email it". |
invoice_reminder (Payment reminder, before due date) | By the daily run "Unpaid reminder, days before the due date" days before the due date, once, for invoices issued before that point; or from the invoice page. |
invoice_overdue_1, _2, _3 (overdue notices) | By the daily run "First", "Second" and "Third overdue notice, days after" days after the due date (0 turns a notice off), each once; only the latest notice due is sent. Clients with "no overdue notices" on their profile get none. Or from the invoice page. |
payment_received (Payment received) | When a payment is recorded with the box ticked, by gateways, and when a renewal invoice is paid in full by credit. Staff with account notices get admin_payment_received. |
payment_failed | When a saved payment method could not be charged. |
refund_issued (Refund issued) | When a refund is made with "Email the client" ticked. |
Reminders need "Send payment reminders" (Settings > Automation Settings, Billing) and go only to invoices that still have a balance. The reminders sent are remembered per invoice, so none is sent twice.
Late fees
When "Add the late fee this many days after" (Automation Settings, Billing) is above 0, the daily run adds one "Late payment fee" line to each open invoice that many days past its due date and has no late fee yet. The fee is Late fee type and Late fee from the Invoices tab: a percentage of the invoice total, or a fixed amount in the default currency converted to the invoice's currency; never less than Smallest late fee (also converted). It is taxed when "Tax late fees" is on. Clients marked exempt from late fees on their profile get none.
Automatic cancelling
"Cancel invoices unpaid for this many days (0 = never)" (Automation Settings, Billing): the daily run cancels unpaid invoices that many days past due that have no payments and no credit on them.
Transactions
A transaction is money in or out: a payment on an invoice, a refund, a deposit to credit, or money paid out. Each has a client, a date, a payment method, an optional reference (the gateway's transaction id or your own), amount in, fees, amount out, the exchange rate of the day and a description.
Billing > Transactions List
Permission transactions.list. Filters: reference or description, method, from and to dates, and a client filter from the client's page. Columns: date, client, method, reference and description, invoice, in, fees, out. Under the table, the totals in, fees and out per currency.
Add Transaction
Billing > Transactions List > Add Transaction (transactions.add) records money that is not entered on an invoice page:
| Field | What it does |
|---|---|
| Client number | Required. |
| Date | The day of the payment. |
| Payment method | A gateway or manual method (Bank transfer, Cash, Cheque, Other). |
| Reference (optional) | A payment with the same method and reference as an existing one is refused. |
| Money received, Money paid out | Enter one or the other, not both. |
| Fees | Fees taken by the payment provider. |
| Invoice number (optional) | Money received for one of the client's invoices is recorded as a payment on it, with everything that paying means. Money paid out cannot go on an invoice; refund it from the invoice instead. |
| Description | Free text. |
| Add the money received to the client's credit (when it is not for an invoice) | A deposit: the amount is added to the client's credit. |
Reversal
A payment recorded by mistake, or one the bank sent back, is reversed from the invoice page (Reverse next to the payment). The remaining amount of the payment is recorded as going out again; any overpayment credit it created is taken back off the client's credit (as far as the balance allows); if the payment was what paid the invoice, the invoice becomes unpaid again and the services, add-ons and domains it moved on go back one period, unless a later paid invoice has moved them on again. A payment already refunded or reversed cannot be reversed.
Payments
Recording a payment
From the invoice page (Add payment) or Add Transaction with an invoice number. The amount must be above zero. Partial payments are allowed: the invoice stays open until the balance (total less credit less payments) is covered. Money beyond the balance, and any payment added to an invoice that is already paid, goes to the client's credit as "Overpayment of invoice #N". Payments on cancelled, refunded and draft invoices are refused. A payment with a gateway and reference already recorded is refused, so a provider repeating itself never records twice.
What paying an invoice does
When the balance is covered, in this order:
- The invoice becomes paid, with the payment method that paid it; with "Set the invoice date to the payment date" the issued date changes; the sequential number is given (unless given at creation).
- Each service and add-on line moves its item on by one billing cycle, and each domain line by the domain's years, but only when the item's next due date still equals the period start stored on the line, so paying the same period twice never moves an item twice. The next invoice date is moved up to at least the new due date.
- An add-funds line adds its amount to the client's credit.
- Each "Invoice #N" line of a mass-pay invoice marks that invoice paid.
- Services of products set to "When the first payment arrives" are set up; paid upgrades are applied; services suspended for not paying are unsuspended when "Unsuspend on payment" is on and nothing else of theirs is overdue.
- Domain registrations and transfers on the invoice are sent to the registrar; renewals are sent when "Renew at the registrar when the renewal is paid" is on.
- Affiliate commissions on referred services are recorded.
- The payment confirmation email goes to the client and the staff notice to staff who receive account notices.
Refunds
Refund next to a payment on the invoice page (invoices.refund). Enter the amount (at most what is left of that payment after earlier refunds) and where it goes:
- Add to the client's credit: the client's credit goes up by the amount.
- Send it back through the gateway: offered when the payment's gateway can refund and the payment has a provider reference; the gateway is asked to refund and its reference is kept. If the gateway refuses, nothing is recorded.
- Record a refund I sent myself: only recorded; send the money yourself.
A reference can be typed, and "Email the client" sends refund_issued. The refund is a transaction with the amount out, linked to the payment. When every payment on a paid invoice is refunded, the invoice becomes refunded. Cancel and refund on an order page refunds every payment on the order's invoice the same way and cancels the order (see Orders).
Credit
Credit is a running balance per client in the client's currency. Every change is a row in the credit log with a date, description, who made it and the invoice it concerns. Credit comes from overpayments, refunds to credit, deposits (Add Transaction with the credit box, or the client's Add Funds), credit notes and staff additions; it goes out when applied to invoices, taken back by a reversal or removed by staff. It can never go below zero.
Clients > a client > Credit (/clients/ID/credit; credit.view) shows the balance and the log. With credit.manage: Add credit and Remove credit, each with an amount and a description. On the invoice page: Apply credit and Take credit back (above).
Billable items
Billable items are work or goods to charge a client for: an hour of consulting, a migration, a licence. Billing > Billable Items (billable.view; changes need billable.manage) lists them with filters for client and Show (All, Not yet invoiced, Invoiced), and tick boxes with "Invoice the chosen items now" (items of one client only; the invoice is due in seven days; "Email the invoice" sends it).
| Field | What it does |
|---|---|
| Client number | Required. |
| Charged by | Quantity or Hours. |
| Description | The line text. |
| Quantity or hours | A number such as 1 or 2.5. |
| Price each (client's currency) | The unit price; the line amount is quantity times price, and the description shows the working when the quantity is not 1. |
| When to invoice | Do not invoice; Invoice on the next daily run; Add to the client's next invoice (it joins the client's next renewal invoice, and waits until there is one); Invoice on the due date; Invoice again and again. |
| Due date | The date for "Invoice on the due date" and the first date for repeating items. |
| Repeating: Every, Period, How many times (0 = no end) | For "Invoice again and again": every N days, weeks, months or years, for a number of times or without end. The item's due date moves on each time it is invoiced. |
| Delete | Removes the item; invoices already made are untouched. |
The item's page shows how many times it was invoiced and links to those invoices. Billable lines are taxed when "Tax billable items" is on.
Quotes
Quotes are priced proposals for a client or a prospect (someone who is not a client yet). Billing > Quotes (quotes) lists them with a Stage filter and shows who each is for; prospects are marked. Stages: Draft, Delivered, On hold, Accepted, Lost, Dead.
| Field | What it does |
|---|---|
| Subject | Required. |
| Stage | Set by hand; emailing a draft makes it Delivered, converting makes it Accepted. |
| Valid until | Defaults to 30 days from today; clients cannot accept after it. |
| Client number (leave empty for a prospect) | A client quote is in the client's currency. |
| Prospect details | Name, company, email, phone, address, country and the currency, for someone who is not a client. A prospect needs at least a first name and an email address. |
| Lines | Description, quantity, unit price, discount % and Taxed. A line's amount is quantity times unit price less the discount. Three empty rows are added each time; save for more. |
| Proposal text | Shown above the lines on the quote. |
| Notes for the client | Shown under the lines. |
| Staff notes | Not shown to the client. |
Tax follows the client's country, or the prospect's. Actions on a saved quote: Print; Download PDF (Quote-N.pdf); Duplicate (a new draft dated today, with the validity moved by the same number of days); Email quote (quote_delivery, to the client, or to the prospect's address); Convert to invoice (client quotes only, invoices.create; makes an unpaid invoice due in seven days with a line per quote line, the note "Quote #N: subject", stage Accepted, with "Email it"); View invoice once it has one; Delete (not when it has an invoice; set the stage to Dead instead).
Clients see their quotes (not drafts) under Billing > Quotes in the client area (/client/quotes) with Print and PDF, and can accept a Delivered or On hold quote that is still valid. Accepting makes the invoice, tells staff who receive account notices (admin_quote_accepted), emails the invoice and opens it for payment. Prospect quotes are emailed only; to invoice one, make the prospect a client and choose them on the quote.
Credit and debit notes
Notes document corrections after an invoice was issued. A credit note gives the client credit; a debit note raises an invoice for an extra charge. Billing > Credit and Debit Notes (notes.list, notes.view) lists them with Kind and client filters; New credit note and New debit note need notes.create. The invoice page's Credit note button opens a credit note for that invoice.
| Field | What it does |
|---|---|
| Kind | Credit note or Debit note; fixed once issued. |
| Client number | Required. |
| For invoice (optional) | Must be one of the client's invoices. |
| Lines | Description, amount (above zero) and Taxed. Tax follows the client's country and the tax rules. |
| Notes shown on the note | Printed on the note. |
A note is saved as a draft and can be changed or deleted (notes.delete) while it is one. Issue (notes.manage) makes it final and gives it its number (CN- or DN- and its id):
- Issuing a credit note adds its total to the client's credit ("Credit note CN-N"), and, when it is for an invoice that is still open, applies that credit to the invoice at once. "Email the client" sends
credit_note; it can be sent again later with Email. - Issuing a debit note raises an unpaid invoice due in seven days with the note's lines and the note "Debit note DN-N for invoice #M"; "Email the client" sends the invoice. The note page links to the invoice.
Issued notes are kept; Close marks one as dealt with. Print gives a printable page.
Disputes
Billing > Disputes records chargebacks and payment disputes raised with a gateway. Permission "List transactions" to see, "Disputes" to record and decide. The page has tiles (open, to answer within 7 days, won, lost), tabs by status and a table with the client, invoice, gateway, amount, reason, when it was opened, the date to respond by (red when near, "Late" when past) and the status. "Record a dispute" opens a pop-up: find the client by name, email or number, pick the payment (which fills the invoice and amount), and enter the gateway reference, reason and respond-by date. A dispute page shows the facts, the linked invoice and payment, an evidence box (what you sent the provider) and notes, with Under review, Mark won, Mark lost and Withdrawn. Losing records a reversing transaction for the amount and puts the invoice back to unpaid when that leaves it short; winning or withdrawing changes nothing on the invoice. Stripe (charge.dispute.created and closed) and PayPal (CUSTOMER.DISPUTE.CREATED and RESOLVED) webhooks open and close disputes by themselves. Every new dispute emails the staff who receive account notices (template "Dispute opened").
On every billing page opened for one client (?client=: the invoice, transaction, quote, billable item and note lists and their Add forms) a strip under the title shows that client: initials, name, company, email, status, credit, unpaid invoices with the amount outstanding, open disputes, and links to the profile and the related lists. The Add forms preselect and lock the client, with "Choose another client" to change it.
Add funds
Clients deposit money into their credit from Billing > Add Funds in the client area (/client/add-funds). The page shows their balance and credit history; the form makes an invoice with one "Add funds to your account" line (kind add_funds), due today, which opens for payment. Paying it adds the amount to their credit. Such an invoice is never paid with credit and cannot be part of a mass payment. Settings on General Settings, tab Credit:
| Setting | What it does |
|---|---|
| Clients may add funds | Turns the page on. |
| Smallest deposit, Largest deposit | The allowed amounts, in the default currency and converted to the client's. |
| Largest credit balance | The client's credit plus deposits still unpaid may not go above this; the largest deposit is reduced to the room left. |
| Only clients with an active order may add funds | Clients without an Active order are told to come back later. |
| Use credit on new invoices automatically | See Credit on invoices. |
| Give credit for downgrades | Belongs to the services side. |
Mass pay
With "Clients may pay several invoices at once" on, the client's invoice list offers tick boxes on unpaid and collections invoices and "Pay ticked invoices together". CoreInto makes one new invoice whose lines are "Invoice #N" for each ticked invoice's balance (two at least), due on the earliest of their due dates, with the first invoice's payment method and the note "Pays invoices ...". Paying it marks every one of them paid. Add-funds invoices and invoices already covered by an open mass-pay invoice are left out. The mass-pay invoice is one invoice with one payment; it is not split into a payment per invoice.
Settings > Tax Configuration
Tax has two levels, each with rules by country and state. The Tax Configuration page holds the switches; the Tax Rules button on it opens the rules (also at /settings/tax-rules). Permission settings.tax.
| Setting | What it does |
|---|---|
| Charge tax | Master switch. Off: no tax anywhere, whatever the rules. |
| Prices include tax | On: the amounts you set already contain the tax, and the invoice shows the tax taken out of them. Off: tax is added on top. |
| Level 2 tax is charged on top of level 1 | Compound: the second tax applies to the amount plus the first tax. |
| Work out tax per line (otherwise on the total) | Rounding per line, or once on the sum of the taxed lines. |
| Tax domains | Domain lines are taxed. |
| Tax late fees | Late fee lines are taxed. |
| Tax billable items | Billable item lines are taxed. |
| Tax lines added by hand | New lines on invoices, and lines of invoices written by hand, start with Taxed ticked. |
| Name of the client's tax number | The label of the tax number field, for example "VAT number". |
| Ask clients for their tax number | Shows the field on sign-up and the profile. |
| Your own tax or VAT number | Printed on invoices. |
Tax rules
Each rule has a level (1 or 2), a name (such as VAT or GST), a country or "All countries", an optional state or province, and a rate in percent. For each level, the rule that fits the client best applies: a rule for their state wins over a rule for their country, which wins over a rule for all countries. One rule per level and place; delete a rule to change it. Clients marked tax exempt on their profile pay no tax. Products and add-ons have their own "Tax is charged" switch, so an untaxed product adds untaxed lines even for a taxable client. The tax names and rates are stored on each invoice when it is made, so later rule changes do not alter existing invoices.
Renewal dates and proration
Every service, add-on and domain carries two dates:
- Next due date: the first day of the period that is not paid yet. On a new service it is the order day (or the pro rata billing day). When a paid invoice covers that period, the next due date moves on by one cycle (one year times the years for domains). Suspension and termination for non-payment count their days from the due date of the unpaid invoice for that period.
- Next invoice date: the start of the next period that has not been invoiced yet. The daily run invoices a period when this date falls within the invoicing window and then moves it on, so a period is never invoiced twice.
Each invoice line that pays for a period stores that period's start date. That is how paying moves the right item on exactly once, how a reversal moves it back, and how the renewal run knows a period is already on an invoice that is not cancelled.
The invoicing window is set on Settings > Automation Settings, tab Billing: "Make invoices this many days before the due date" (default 14), with separate numbers for monthly, quarterly, half-yearly, yearly, two-yearly and three-yearly services and for domains (blank uses the general one). Each day the task create-invoices collects, per client, every active or suspended service and add-on and every active or grace domain with auto-renew on whose next invoice date is within its window, plus billable items that are due, and makes one invoice per client due on the earliest period start, emails it (invoice_created, or payment_received when credit paid it in full) and moves the next invoice dates on. Clients, or client groups, set to "a separate invoice for each item" get one invoice per item instead. With "Invoice the next period even if the last one is unpaid" (General Settings, Invoices) off, an item with an open invoice is not invoiced again until that is paid; on, up to a year of periods can be invoiced in one run when the window reaches that far.
Pro rata products (Products/Services, tab Other) bill every service on one day of the month: the first invoice charges the share of the period up to that day, and the service's next due date is that day; when the order comes after the "also pay the next month" day, the first invoice includes a whole period as well. Add-ons with "Bill it up to the service's next due date" are prorated to their service's date.
Automation
The daily billing tasks are create-invoices (renewal invoices, above), invoice-reminders (reminders and overdue notices), late-fees, invoice-auto-cancel and capture-payments (charging saved payment methods before the due date), plus the daily exchange-rates. Their settings are on Settings > Automation Settings; each task's last result and a "run now" button are on Utilities > Automation Status. See Automation.
The client's view
Clients see their invoices under Billing > Invoices in the client area (/client/invoices, tabs All, Unpaid, Paid, Cancelled, Refunded, with the amount due and the mass pay tick boxes), one invoice with Print, PDF, the lines, payments, their credit to apply and the payment methods offered, Quotes (/client/quotes) and Add Funds (/client/add-funds). Drafts are never shown. See Client area.
