The Secretary of Defense shall authorize the officials responsible for operation of commissaries and military exchanges to negotiate directly with private carriers for the most cost-effective transportation of commissary and exchange supplies to destinations outside the continental United States without relying on the Air Mobility Command, the Military Sealift Command, or the Military Traffic Management Command. Section 2631 of this title, regarding the preference for vessels of the United States or belonging to the United States in the transportation of supplies by sea, shall apply to the negotiation of contracts for sea-borne transportation under the authority of this section.