49 USC 41911 - Evidence of providing mail service

When and in the form required by the United States Postal Service, an air carrier transporting or handling
(1) United States mail shall submit evidence, signed by an authorized official, that the transportation or handling has been provided; and
(2) mail of a foreign country shall submit evidence, signed by an authorized official, of the amount of mail transported or handled and the compensation payable and received for that transportation or handling.