Here's an email sample you may use to send a polite reminder about an upcoming invoice:
Subject line: Invoice [invoice number] for [product or service] is due on [due date].
Hello [Name],
I hope you are well. Please see attached invoice [invoice number] for [product or service], payable on [due date]. If you have any questions, feel free to contact me.
Sincerely,
[Your name]
You may also provide other information, such as mode of payment.