Three invoice reminders for three different moments.

Subject: A quick note about invoice [number] Hi [Client name], I hope you are well. I am checking in on invoice [number] for [amount], due [date]. Could you let me know when I can expect payment? If you need another copy of the invoice, I am happy to send it. Thank you, [Your name]

Subject: Payment needed: invoice [number] Hi [Client name], Invoice [number] for [amount] was due on [date] and is still unpaid. Please arrange payment today, or reply with the payment date. Let me know right away if there is a problem with the invoice. Thank you, [Your name]

Subject: Final payment request: invoice [number] Hi [Client name], This is my final payment reminder for invoice [number] for [amount], due [date]. Please send payment or reply by the next business day with a firm payment date. I will need to pause further work until this is resolved. Regards, [Your name]