39 U.S. Code § 5001 - Provisions for carrying mail

The Postal Service shall provide for the transportation of mail in accordance with the policies established under section 101(e) and (f) of this title and the provisions of this chapter. Notwithstanding any other provision of this title, the Postal Service may make arrangements on a temporary basis for the transportation of mail when, as determined by the Postal Service, an emergency arises. Such arrangements shall terminate when the emergency ceases and the Postal Service is promptly able to secure transportation services under other provisions of this title.