Subject: Invoice Due: Reminder | [COMPANY_NAME]
This is just a quick reminder that Invoice #[INVOICE_NUMBER] will be due on [DUE_DATE]. I have attached a copy of the invoice to this email.
You can make the payment to the account specified in the invoice. If you want to pay us via other means please contact 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.