10 USC 2649 - Civilian passengers and commercial cargoes: transportation on Department of Defense vessels

(a) Whenever space is unavailable on commercial lines and is available on vessels operated by the Department of Defense, civilian passengers and commercial cargo may, in the discretion of the Secretary of Defense, be transported on those vessels. Rates for transportation under this section may not be less than those charged by commercial lines for the same kinds of service.
(b) Amounts received under this section shall be covered into the Treasury as miscellaneous receipts.