Basics
Invoice mistakes that delay payment, and how to avoid each one
The small errors that send an invoice back or leave it waiting: no due date, the wrong name, a missing order number, vague lines, tax slips and no way to pay. What each one costs and the fix.
26 Oct 2026 · 7 min read
Lena sends an invoice for €3,570 to a new client in Munich. Three weeks later she asks when to expect the money. The reply: the invoice was never booked, because it named “Kestrel Studio” and their company is “Kestrel Studio GmbH”, and it had no purchase order number. Nobody told her. The work was fine. The invoice was not quite right, and that was enough to stop it.
Most late payments start like this, with an ordinary gap rather than a customer who refuses to pay. Here are the common ones, what each costs you, and how to avoid it.
The mistakes at a glance
| Mistake | What happens | The fix |
|---|---|---|
| Sent days or weeks after the work | The payment clock starts late | Invoice the day the work is done |
| No due date, or only “Net 30” | Nobody knows when it is late | Print the actual date |
| The wrong company name or address | Their accounts team cannot book it | Use the legal name from their order or contract |
| No purchase order number | It waits in a queue until someone matches it | Ask for the PO before you start |
| Lines nobody can check | Questions, then a delay | Say what, how much and when, on each line |
| Tax charged wrongly, or your tax number missing | Rejected, or a correction later | Check registration, rate and numbers |
| No clear way to pay | The payer has to write and ask | Bank details and a link on every invoice |
| Sent to the wrong person | It sits in an inbox that does not pay bills | Ask who pays, and where invoices go |
| A sum that does not add up | Doubt about the whole invoice | Let software do the sums, or check twice |
1. Sending it late
Payment terms count from the invoice date, not from the day you finished. An invoice sent two weeks after the job, on 30-day terms, is paid six weeks after the job at best. Send it the same day, while the client still remembers the work and is pleased with it. Our post on when to send an invoice covers deposits, month-end billing and other timings.
2. No due date
“Payment within 30 days” leaves the reader to do the arithmetic, and “due on receipt” is read by many as “whenever”. Write the date itself: “Due 15 November 2026”. Then there is no doubt about when it is late, and a reminder the day after is fair. See invoice payment terms.
3. The wrong name or address
You know your contact as “Sam at Kestrel”. Their accounts team knows the company by its registered name and address, and often by its tax number. An invoice made out to a trading name, a person or an old address may be returned, or quietly parked. Ask for the billing details before the first invoice, copy them exactly, and keep them on the customer's record so the next invoice is right too.
4. No purchase order number
Larger companies pay only against an approved order. If their order has a number and your invoice does not, someone has to find the match by hand, and that someone is busy. Put the PO number near the top of the invoice, exactly as they wrote it. More in purchase order numbers on invoices.
5. Lines nobody can check
“Design work, £2,400” asks the client to trust you. “Homepage and four inner pages, two rounds of changes, 3 to 14 October” lets them tick it off. The person approving the invoice is often not the person who asked for the work, so write for them. Our guide to writing invoice line items has examples.
6. Tax slips
- Charging tax when you are not registered. You cannot collect VAT, GST or sales tax you are not registered for. If you are not, say nothing about tax, or the note your country expects.
- Leaving out your tax number. A business customer usually needs it to claim back the tax. Without it, the invoice may not count.
- The wrong rate, or the wrong kind of tax. In India, CGST and SGST for a customer in your state and IGST for one in another. In the EU, a reverse charge for a business customer in another member state.
- Missing the customer's tax number where it is needed, such as on a reverse charge invoice.
What each country's invoice must show is in our checklist of what every invoice must show.
7. No clear way to pay
If the payer has to write back to ask for your bank details, you have added a day at least. Print them on every invoice: account name, number, bank code, and the SWIFT or IBAN for clients abroad. A payment link is even easier. Ask them to quote the invoice number as the reference, so you can tell which invoice the money is for.
8. Sent to the wrong person
Your contact may forward it, or may not. Many companies want invoices at one address, like accounts@, and only process what arrives there. Ask once, “Where should invoices go, and who approves them?”, and send to both if needed.
9. Sums that do not add up
A total that is a few cents off, or a tax line that does not match the rate, makes a careful payer check everything else too. Templates with hand-typed formulas break quietly when a row is added. Software that works out each line and the tax avoids this. Small rounding differences are normal, and why totals differ by a cent explains them.
10. Fixing a sent invoice by changing it
Once an invoice has gone, your client may have booked it. If you edit it and send it again under the same number, your copy and theirs disagree. Cancel it and issue a new one, or issue a credit note for the difference. See cancelling an invoice.
11. Not following up
Some invoices are simply forgotten. A short, polite reminder a few days before the due date and another the day after it solves more late payments than any late fee. If nobody chases, the invoice drifts to the bottom of the pile.
The short version
- Send the invoice the day the work is done, with the due date written as a date.
- Use the customer's legal name and address, and their PO number if they have one.
- Describe each line so someone who was not there can approve it.
- Check your tax registration, the rate and both tax numbers.
- Print how to pay, and ask for the invoice number as the reference.
- Never change a sent invoice. Cancel and reissue, or issue a credit note.
- Remind before the due date and the day after.