I wouldn't get to paranoid. Many of the carriers routinely check for illegal drugs at the terminals. But they are usually looking for the kind of stuff dogs can smell. UPS, FedX and most of the private carriers have the right to inspect any package without a warrant. There is an old law enforcement trick where the envelope is sprayed with a non residue type cleaner it makes the viewing the inside of the package even reading a letter if it is not folded easy. When the cleaner "alcohol" based evaporates you would never know. Boxes are another story.
Take care. Billylll