39 U.S.C. § 603 - U.S. Code - Unannotated Title 39. Postal Service § 603. Searches authorized

(1) vehicle passing, or having lately passed, from a place at which there is a post office of the United States;

(2) article being, or having lately been, in the vehicle; or

(3) store or office, other than a dwelling house, used or occupied by a common carrier or transportation company, in which an article may be contained.