Pick a consistent pattern
A sequence like INV-001, INV-002 and INV-003 is easy to maintain. If you prefer a year prefix, use a pattern such as INV-2026-001. Keep the format consistent in filenames and payment references.
Keep your own register
Record the number, customer, issue date and amount in a spreadsheet or bookkeeping system. This local tool does not reserve numbers or detect duplicates across devices.
Do not quietly reuse a sent number
If you need to correct a document, keep a record of what changed and reference the original. Confirm any numbering and correction requirements that apply to your own business before adopting a system.