39 U.S. Code § 5007 - Free transportation of postal employees
Free transportation of postal employees
Each person or carrier engaged in the transportation of mail shall carry on any vessel, train, motor vehicle, or aircraft he operates, upon exhibiting their credentials and without extra charge therefor, persons on duty in charge of the mails or when traveling to and from such duty.
In this subsection, “air carrier” and “aircraft” have the same meanings given those terms in section 40102(a) of title 49.
An air carrier engaged in transporting mail shall carry without charge on any plane it operates those agents and officers of the Postal Service traveling on official business related to transporting mail by aircraft, as prescribed by regulations of the Secretary of Transportation, on exhibiting credentials.