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. Clearly state the decision regarding the refund. Demand letters: Upon detection of a potential overpayment, Medicare or Medicaid will send a demand letter requesting payment. There are instances where you as the child custody paying parent may send a child support payment that exceeds the court ordered amount.