41 C.F.R. § 102-118.145

Agencies must choose a method auditing transportation invoices:

(a) Agencies are encouraged to consider using a third-party electronic payment processor for transportation invoice processing, payment, and prepayment audit.

(b) Create an internal prepayment audit program.

(c) Contract directly with a prepayment audit service provider.

(d) Use the services of a prepayment audit contractor under GSA's multiple award schedule covering audit services, including transportation prepayment audit services (541211 Auditing Services).

(e) Use a third-party payment system (TPPS) or charge card company that includes prepayment audit functions.