39 U.S.C. § 5002: Transportation of mail of adjoining countries through the United States

The Postal Service, with the consent of the President, may make arrangements to allow the mail of countries adjoining the United States to be transported over the territory of the United States from one point in that country to any other point therein, at the expense of the country to which the mail belongs, upon obtaining a like privilege for the transportation of United States mail through the country to which the privilege is granted.