I expect the day will come when UPS and other shippers start installing "sniffers" in their depots. When that happens if what you noted to be in the contents turns out to be something completely different you'll probably get a visit from either the ATF, FBI, or DEA. I would also expect there would be some Federal Felony charges to deal with.
As for why I think this will happen, it's pretty simple. Criminals are currently shipping all kinds of nastiness and in the interest of National Security I expect we will see some Laws passed requiring "sniffers" and Felony charges for content violations.
As for what kinds of nastiness is probably being shipped right now. First, just about any legal or illegal drug you can think of. Second, chemicals and compounds that can be used to make bombs or explosives. Third, chemicals and compounds that can be used to make poison's from Sarin to Mustard Gas. One thing about the Net is that it disseminates information to a wide audience and that information can be either good or bad.