Complete form CAT REF. • Amended return overpayment letter may serve as substitute.Below are the ways you can submit a repayment. I overpaid my estimated taxes. How do I apply for a refund? In order for an overpayment refund to be processed in a timely manner, please submit a completed form with all refund checks and supporting documentation. Open the Application for Refund; you may fill it out online, but then you must print the completed form. Sign the form in the presence of a Notary Public. Need to complete Computation of Overpayment Worksheet. When you identify a Medicare overpayment, use the Overpayment Refund Formto submit the voluntary refund.