Most late invoices are not refusals. They are forgotten, stuck in approval, or sent to the wrong person. A short, friendly reminder on the right days gets most of them paid. Always send to the person who pays, not only the person who ordered, and attach the invoice again.
1. The day after it is due
Subject: Invoice [number] for [amount]
Hi [name],
A quick note that invoice [number] for [amount] was due yesterday. I have attached it again in case it got buried. You can pay by [bank details or link]. If it is already on its way, thank you, and please ignore this.
[Your name]
2. A week later
Subject: Following up: invoice [number]
Hi [name],
I am following up on invoice [number] for [amount], now 7 days overdue. Could you let me know when it will be paid, or whether anything is holding it up, such as a missing PO number or an approval? I am happy to fix that today.
[Your name]
3. Two weeks after due
Subject: Invoice [number] is 14 days overdue
Hi [name],
Invoice [number] for [amount] is now two weeks past due. Please confirm a payment date by [date]. If there is a problem with the invoice, tell me what it is and I will sort it out right away. If useful, we can agree a payment plan.
[Your name]
4. Final notice, 30 days
Subject: Final reminder: invoice [number]
Hi [name],
This is my final reminder for invoice [number] for [amount], 30 days overdue. Under our terms a late fee of [rate] applies. Please pay by [date], or call me today so we can agree a plan. After that date I will have to pause further work and look at other options.
[Your name]
What makes these work
Be specific about the amount and date. Make paying easy, with the link or bank details in every email. Give a next step, not a threat. Keep a record of every message. Never contact someone at odd hours or in a way that embarrasses them, and never threaten action you will not take.
This is general guidance, not legal advice. Check your contract and local rules before adding fees or taking further action.