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. We have identified the following overpayment and are in good faith voluntarily refunding all monies collected in error. The salary overpayment resulted because . Overpayment Information. If you have received an overpayment notice from DETR, you may be required to repay the excess amount.