39 U.S. Code § 2602 - Transportation of international mail by air carriers of the United States
(a) The Postal Service may offset against any balances due another country resulting from the transaction of international money order business, or otherwise, amounts due from that country to the United States, or to the United States for the account of air carriers of the United States transporting mail of that country, when—
(1) the Postal Service puts into effect rates of compensation to be charged another country for transportation; and
(b) When the Postal Service has proceeded under authority of subsection (a) of this section, it shall—
(2) 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
(c) The Postal Service may advance to an air carrier, out of funds available for payment of balances due other countries, the amounts determined by the Postal Service to be due from another country to an air carrier for the transportation of its mails when—
(2) the Postal Service determines that the balance of funds available is such that the advances may be made therefrom.
Collection from another country of the amount so advanced shall be made by offset, or otherwise, and the appropriation from which the advance is made shall be reimbursed by the collections made by the United States.
(d) 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.