UPS and Fedex can do this as private companies, my understanding is the USPS actually can't due to the 4th Amendment: "The Fourth Amendment protects the privac

UPS and Fedex can do this as private companies, my understanding is the USPS actually can't due to the 4th Amendment: "The Fourth Amendment protects the privacy of U.S. mail as if it had been kept by the sender in his home. Ex parte Jackson, 96 U.S. 727, 732-733, 735 (1878); U.S. v. Van Leeuwen, 397 U.S. 249, 252-52 (1970). It cannot be searched without a search warrant based on probable cause."