The GSA Transportation Audits Division does not charge agencies a fee for conducting the transportation postpayment audit. Transportation postpayment audits expenses are financed from overpayments collected from the TSP's bills previously paid by the agency and similar type of refunds. However, if a postpayment audit is conducted in lieu of a prepayment audit at the request of an agency, or if there are additional services required, GSA may charge the agency.