Subject: Payment of outstanding balance | [COMPANY_NAME]
This is a reminder that you have an outstanding balance of [OUTSTANDING_AMOUNT] that was due on [DUE_DATE] with us. I have attached a copy of the invoice to this email.
I would appreciate if you could treat this matter with utmost priority and settle the dues immediately.
In the event that you have made the payment, please disregard this email and accept my sincere apologies for the trouble.
If you need more details, please write back to me or call me on my number listed below.