Mailing Standards of the United States Postal Service - International Mail Manual > 7 Treatment of Inbound Mail > 780 Items Mailed Abroad by or on Behalf of Senders inthe United States
Payment of U.S. postage is required to secure delivery of mail when the mailing is by or on behalf of a person or firm that is a resident of the United States and the foreign postage price applied to such items is lower than the comparable U.S. domestic rate.
Undeliverable-as-addressed mail returned to the sender for which outbound postage was not paid to the U.S. Postal Service is subject to the payment of handling charges. On delivery to the sender, the sender is charged the applicable First-Class Mail International or First-Class Package International Service price for the weight and shape of the returned piece.