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.You have to tell your credit or debit card company about any error you're disputing within 60 days of when the first statement with the charge was sent to you. We have identified the following overpayment and are in good faith voluntarily refunding all monies collected in error. A demand letter is a letter you can send to try to resolve a dispute before going to court. Demand letters are often sent in cases where someone owes money. 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.