39 U.S.C. § 5005

(a) The Postal Service may obtain mail transportation service— (1) from air carriers as provided in chapter 54 of this title; (2) from water carriers as provided in chapter 56 of this title; and (3) by contract from any person or carrier for surface and water transportation under such terms and conditions as it deems appropriate, subject to the provisions of this section. (b) (1) Contracts for the transportation of mail procured under subsection (a)(3) of this section shall be for periods not in excess of 4 years (or such longer period of time as may be determined by the Postal Service to be advisable or appropriate) and shall be entered into only after advertising a sufficient time previously for proposals. The Postal Service, with the consent of the holder of any such contract, may adjust the compensation allowed under that contract for increased or decreased costs resulting from changed conditions occurring during the term of the contract. (2) A contract under subsection (a)(3) of this section may be renewed at the existing rate by mutual agreement between the contractor or subcontractor and the Postal Service. (3) Any contract between the Postal Service and any carrier or person for the transportation of mail shall be available for inspection in the office of the Postal Service and either the Surface Transportation Board or the Secretary of Transportation if for the carriage of mail in foreign air transportation (as defined in section 40102(a) of title 49 ), as appropriate, and in post offices on the post roads involved as determined by the Postal Service, at least 15 days prior to the effective date of the contract. (c) The Postal Service, in determining whether to obtain transportation of mail by contract under subsection (a)(3) of this section or by Government motor vehicle, shall use the mode of transportation which best serves the public interest, due consideration being given to the cost of the transportation service under each mode.