Common Invoice Mistakes and How to Avoid Them
Most late payments are not caused by unwilling clients. They are caused by invoices that cannot be processed as received. Each mistake below adds days, and several of them add weeks.
Mistakes that stop payment outright
| Mistake | What happens | Fix |
|---|---|---|
| Billed to the wrong legal entity | Accounts payable rejects it | Copy the entity name from the contract |
| Missing purchase order number | Invoice sits unmatched in a queue | Request the PO before starting work |
| Duplicate invoice number | Flagged as a possible double billing | Keep a single sequential register |
| No due date | Invoice drops to the bottom of the pile | State a calendar due date, not just Net 30 |
| Editable file format | Rejected or altered | Always send PDF |
| Sent only to your day-to-day contact | Never reaches accounts payable | Copy the AP address on every send |
Calculation and tax errors
The most frequent arithmetic error is applying tax before a discount, which inflates the tax figure. The standard order is subtotal, discount, tax on the discounted amount, then shipping.
Rounding is the second: round once at the total, not on every line, and make sure the printed line totals actually add up to the printed subtotal. A client who finds a one-unit discrepancy will re-check everything else.
If you are registered for GST or VAT, an invoice missing your registration number may not be usable by the client to reclaim tax, and they will ask for a reissue.
Description and scope errors
- One-word line items such as "Services" that nobody can approve.
- Charges that never appeared in the quote, with no explanation.
- Combining several months or projects into one unexplained lump.
- No dates on work that was delivered over a period.
- Currency omitted on a cross-border invoice.
Process mistakes
Invoicing late is a self-inflicted delay: an invoice sent three weeks after delivery is paid at least three weeks later, and the approver has already moved on. Invoice within a day of finishing.
Not keeping a copy is the other one. Because this tool runs in your browser and does not store your invoices on a server, download the PDF and keep it in your own folder or drive before you clear the form.
A pre-send checklist
- Correct legal entity, address and accounts contact.
- Unique invoice number and today's issue date.
- Calendar due date matching agreed terms.
- PO or quote reference where one exists.
- Every line item specific and dated.
- Discount applied before tax; totals add up.
- Tax registration number shown if applicable.
- Payment details complete and current.
- Exported as PDF and saved to your own storage.
Frequently asked questions
A client says they never received my invoice. What now?
Resend the same invoice number with the PDF attached, copy the accounts payable address, and ask for written confirmation of receipt. Never issue a new number for a resend.
Can I add a charge I forgot?
Issue a separate additional invoice referencing the original, or credit the original and reissue in full. Do not silently edit a sent invoice.
How long should I keep copies?
Follow your local tax retention period, commonly five to eight years. Keep PDFs in dated folders with a backup copy.
Sources and further reading
Rules differ by country. Confirm the details for your own situation with the official guidance below before relying on this article.
More reading: how to create an invoice, invoice numbering, payment terms, invoices vs quotations, common mistakes and small business invoicing. Practical questions are answered on the FAQ page.
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