39 U.S.C. § 2602: Transportation of international mail by air carriers of the United States

the Postal Service puts into effect rates of compensation to be charged another country for transportation; and the United States is required to collect from another country the amounts owed for transportation for the account of the air carriers. give appropriate credit to the country involved; pay to the air carrier the portion of the amount so credited which is owed to the air carrier for its services in transporting the mail of the other country; and deposit in the Postal Service Fund that portion of the amount so credited which is due the United States on its own account. collections are to be made by the United States for the account of air carriers; and the Postal Service determines that the balance of funds available is such that the advances may be made therefrom. If the United States is unable to collect from the debtor country an amount paid or advanced to an air carrier within 12 months after payment or advance has been made, the United States may deduct the uncollected amount from any sums owed by it to the air carrier. The Postal Service shall adopt such accounting procedures as may be necessary to conform to and carry out the purposes of this section.