Inside an innocuous parcel sits a camera, peering out of a hole to snap images every ten seconds as it is shipped through the postal system. It's the "Spy Box", a project by UK artist Tim Knowles. The resulting 6,994 images were edited together to make a movie which was displayed alongside the Spy Box in the gallery to which the parcel was shipped.
If the Spy Box were to be sent through the American postal system, its images would almost certainly be in violation of some rule or restriction. (Not that it would make it through without being sent to the bomb disposal hopper.)
There's a whole discipline within pop art that uses the postal system as its conduit, from attempts to mail strangely shaped items to shows that make the mailbag as important as the art.