39 U.S.C. § 3401 Mailing privileges of members of Armed Forces of the United States
(a)(1) First-class letter mail having the character of personal correspondence shall be carried, at no cost to the sender, in the manner provided by this section, when mailed by an eligible individual described in paragraph (2) and addressed to a place within the delivery limits of a United States post office, if—(A) such letter mail is mailed by the eligible individual at an Armed Forces post office established in an overseas area designated by the President, where the Armed Forces of the United States are deployed for a contingency operation as determined by the Secretary of Defense; or (B) the eligible individual is hospitalized as a result of disease or injury incurred as a result of service in an overseas area designated by the President under subparagraph (A). (2) An eligible individual described in this paragraph is—(A) a member of the Armed Forces of the United States on active duty, as defined in section 101 of title 10; or (B) a civilian employee of the Department of Defense or a military department who is providing support to military operations.
(b) There shall be transported by surface or air, consistent with the service purchased by the mailer, between Armed Forces post offices or from an Armed Forces post office to a point of entry into the United States, the following categories of mail matter which are mailed at any such Armed Forces post office:(1) Letter mail communications having the character of personal correspondence. (2) Any parcel exceeding 1 pound in weight but less than 70 pounds in weight and less than 130 inches in length and girth combined. (3) Publications published not less frequently than once per week and featuring principally current news of interest to members of the Armed Forces of the United States and the general public.
(c) The Department of Defense shall transfer to the Postal Service as postal revenues, out of any appropriations or funds available to the Department of Defense, as a necessary expense of the appropriations or funds and of the activities concerned, the equivalent amount of postage due, as determined by the Postal Service, for matter sent in the mails under authority of subsection (a) of this section.
(d) The Department of Defense shall transfer to the Postal Service as postal revenues, out of any appropriations or funds available to the Department of Defense, as a necessary expense of the appropriations or funds and of the activities concerned, sums equal to the expenses incurred by the Postal Service, as determined by the Postal Service, in providing air transportation for mail mailed at or addressed to Armed Forces post offices established under section 406 of this title, but reimbursement under this subsection shall not include the expense of air transportation (1) for which the Postal Service collects a special charge to the extent the special charge covers the additional expense of air transportation or (2) that is provided by the Postal Service at the same postage rate or charge for mail which is neither mailed at nor addressed to an Armed Forces post office.
(e) This section shall be administered under such conditions, and under such regulations, as the Postal Service and the Secretary of Defense jointly may prescribe.
(f) In this section:(1) The term “military aircraft” means an aircraft owned, operated, or chartered by the Department of Defense. (2) The term “United States air carrier” has the meaning given the term “air carrier” in section 40102 of title 49.