Subject: Invoice Due: Reminder | [COMPANY_NAME]
This is a quick reminder that the invoice bearing #[INVOICE_NUMBER] is due today. I have attached a copy of the invoice to this email.
You can make the payment to the account mentioned in the invoice. If you want to pay via other means please write back to us.
In the event that you have made the payment, please disregard this email and accept my sincere apologies for the trouble.
Please let me know if you have any questions about the invoice.