Updated 6 September 2026
Why invoice numbers have to be sequential
An invoice number is the handle AP, you, and (if you are VAT-registered) HMRC use to know this bill is not that bill. Gaps and duplicates show up at year end.
Last Friday’s Word file does not own a sequence. Save As, change the client, forget to bump INV-3, email a PDF. Three weeks later you cannot tell which version they paid. That is the whole reason an invoice app exists for a solo operator — not a chart of accounts.
Rules in plain language
UK VAT invoices generally need a unique sequential number. CRA GST/HST ITC rules care more about identity, tax, and amounts than about your numbering scheme — but AP still rejects mystery PDFs. The IRS does not mandate a federal invoice template; US clients still want a number they can put on a remittance.
Sequential means you can explain the series. Skipping numbers because a draft was deleted is a mess you will explain once. Better: drafts consume a number, or you void rather than pretend INV-0004 never existed.
See HMRC VATREC5010 for UK particulars. This page is not tax advice.
What justIssued does
Each justIssued account has a next invoice number and a next estimate number. New documents take the next value. Share pages and print/PDF show that number. This is on Free — it is table-stakes, not an upsell.
Estimates use an EST- sequence and convert to an invoice with a new INV- number when you accept the work (Paid). Recurring templates mint invoices on the same sequence when they run.
Related: how to invoice as a freelancer in Canada and Word vs an invoice app.
justIssued is the billing loop, not a second set of books.
Numbered PDF, share link, outstanding. Free is 3 clients. Paid is $12/mo after a 7-day trial (card required). No Pay now. No cut of client payments.