I was thinking of what you can and cannot mail; specifically, that what he did could be construed as a letter bomb. Granted, it’s much much less dangerous, but the mechanism is the same. At a time when public figures get pipe bombs in the mail, rigging a package to “explode” once opened... I think there’s enough material for a humorless DA (or one with an agenda) to make hay, or at least for a uneasy explanation with some serious people. Obviously IANAL, and I don’t know if the federal laws on mail extend to packages left on the porch as the other poster pointed out, but still.

Source: what my teenage self thought was a hilarious prank got to the attention of the FBI. Although the matter was cleared up in the end, being interviewed by an FBI agent at 17 taught me not to count on the understanding and second degree of law officers.