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.Report an overpayment. Complete an overpayment refund form to notify First Coast of an overpayment prior to receiving a demand letter. Fee adjustments must be made prior to the refund request to reflect an overpayment of the amount being refunded. A detailed explanation of how the refund amount was computed;. • The specific reason(s) for the refund request;. Only petitions for single parcels may be filed online. 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 IRS uses Letter 3286C to inform you that they believe you may have overpaid for a particular year and will refund you the difference.