41 CFR 301-52.10: -52.10 Late payment fee calculation.

(a) To calculate late payment fees, the agency must either—

(1) Use the prevailing Prompt Payment Act Interest Rate beginning on the 31st day after submission of a proper travel claim and ending on the date on which payment is made; or

(2) Reimburse a flat fee of not less than the prompt payment amount, based on an agencywide average of travel claim payments.

(b) In addition to the fee required by paragraphs (a)(1) and (2) of this section, the agency must also pay an amount equivalent to the late payment charge that the card contractor would have been able to charge the employee had the employee not paid the bill.