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. The salary overpayment resulted because . A. Did we tell you in the overpayment notice that you are responsible for paying back another individual's overpayment? We have identified the following overpayment and are in good faith voluntarily refunding all monies collected in error. The Notice of Overpayment will include information on how to appeal. You must appeal the overpayment within 90 days of the date on the notice.