Invoice essentials
Invoice numbering and record keeping for cross-border sales
A useful numbering system is unique, chronological enough to audit and stable across the issued document, accounting record and tax return. The invoice is then retained with the evidence supporting its tax treatment.
Decision summary
- Use one documented unique-number sequence or controlled series.
- Never silently reuse or overwrite an issued number.
- Correct through a traceable credit, cancellation or replacement process.
- Retain tax, export and payment evidence with the invoice.
Choose a numbering scheme that survives growth
A simple sequence such as 2026-0001 is usually easier to audit than a number assembled from customer names, currencies and project codes. Multiple series can be useful for legal entities or systems, but document the ownership and avoid collisions.
Do not put sensitive customer data in the number. Keep the identifier stable even if the customer, amount or payment status changes.
- Unique across the relevant issuing entity
- Generated from a controlled sequence
- Readable without embedding personal data
- Consistent between PDF and accounting system
- Documented treatment of cancelled numbers
Separate issue, supply and due dates
The invoice date identifies when the document was issued. The supply or tax-point date identifies when the underlying transaction is treated as occurring. The due date follows the agreed payment term. These dates can differ and should not be collapsed into one field.
Cross-border tax returns, exchange rates and evidence deadlines may depend on the supply date rather than the due date. Preserve all relevant dates in the record.
Correct errors without erasing history
Once an invoice has been issued, keep the issued version. Use the jurisdiction's credit-note, cancellation or replacement method and cross-reference the original number. Do not edit an archived PDF and pretend it was the original.
A replacement invoice should make the relationship to the prior document clear. The accounting entries and customer communication should tell the same story.
Retain more than the invoice
Keep the contract, purchase order, registration validation, customer-status evidence, export documents, exchange-rate record, correspondence and proof of payment when they support the transaction. The exact retention period and acceptable format depend on the jurisdiction.
Store records with access controls and tested backups. Billr deliberately does not save invoices to a server, so the user must archive the downloaded PDF and evidence in an appropriate business system.
Review point: A browser session is not a books-and-records system. Download, verify and archive every final invoice you issue.
Common questions
Can invoice numbers contain letters?
Many systems permit alphanumeric series, but the number must remain unique and comply with the issuing jurisdiction's sequence rules.
Can I delete a cancelled invoice number?
Keep a trace of cancelled or void numbers and the reason. Silent gaps can make an audit trail harder to explain.
Does Billr save my invoice history?
No. The product is privacy-focused and session-based. Download and archive the final PDF in your own controlled record system.
Primary sources
Use these current public sources to verify the treatment for your transaction.
- Invoices — what they must includeGOV.UK
- GST/HST records to keepCanada Revenue Agency
- Tax invoice rulesCentral Board of Indirect Taxes and Customs