41 CFR § 301-71.210 - How do we calculate late payment fees?
Late payment fees are calculated either by:
(a) 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) 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, you must also pay an amount equivalent to any late payment charge that the card contractor would have been able to charge had the employee not paid the bill. Payment of this additional fee will be based upon the effective date that a late payment charge would be allowed under the agreement between the employee and the card contractor.