41 CFR § 102-118.50
How does my agency pay for transportation services?
June 9, 2020
Your agency may pay for transportation services in three ways:
(a) Electronic funds transfer (EFT) (31 U.S.C. 3332, et). Your agency is required by statute to make all payments by EFT unless your agency receives a waiver from the Department of the Treasury.
(b) Check. For those situations where EFT is not possible and the Department of the Treasury has issued a waiver, your agency may make payments by check.
(c) Cash. In very unusual circumstances and as a last option, your agency payments may be made in cash in accordance with Department of the Treasury regulations (31 CFR part 208).