41 CFR 301-52.20 - How are late payment fees calculated?
Your agency must either:
(a) Calculate late payment fees using 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
(b) Reimburse you a flat fee of not less than the prompt payment amount, based on an agencywide average of travel claim payments; and
(c) In addition to the fee required by paragraphs (a) and (b) of this section, your agency must also pay you an amount equivalent to any late payment charge that the card contractor would have been able to charge you had you not paid the bill.