All mail is subject to Customs examination.

The United States Customs territory consists of the 50 states, the District of Columbia, and Puerto Rico. In general, all mail originating outside United States Customs territory that is to be delivered inside U.S. Customs territory is subject to Customs examination. Mail arriving from outside the U.S. Virgin Islands that is to be delivered inside the U.S. Virgin Islands is also subject to Customs examination.

Mail entering the United States from abroad arrives first at the U.S. Postal Service. The Postal Service then sends packages to U.S. Customs for examination and to assess duty, if indeed any is owed. Customs processing is required for civilian parcels as well as for those sent from overseas military postal facilities (APO/FPO).