This brief but important letter notifies a customer that he or she has overpaid you. It can serve as a very good customer relations letter.Our document template is designed to make the process of writing an "Inform a Customer of an Overpayment" letter quick, easy, and stress-free. Whether the overpayment notification letter to the customer is a system generated letter (Notice 18) or a manual letter it must state the following information:. The salary overpayment resulted because . Whether the overpayment notification letter to the customer is a system generated letter. Topics on this page: What is Financial Hardship? What to Do If You Cannot Pay Your Bills; What is a Hardship Letter?