Coast Guard facilities and personnel may be utilized for the transportation and delivery of mail matter during emergency conditions or at isolated locations under such arrangements as may be satisfactory to the Secretary and the United States Postal Service.
14 U.S. Code § 706 - United States Postal Service
This section provides generally for what has been the practice between the Coast Guard and the Post Office Department in Alaska for years. The authorization is limited to emergency conditions or isolated locations. 81st Congress, House Report No. 557.