This could be something that would certainly be attempted in the future but I bet that the U.S government and others would clamp down on this kind of thing in a second, if you think they hate private and encrypted communication they hate the idea that somebody could be delivering something physical right under their noses is even worse and they'd hunt you down for sure. You wouldn't really be able to do this kind of thing legally, they'd search you anyway if you went to an airport or had a boat for instance to take you across, the only way to do it would be through illegal smuggling.
It's interesting though, we only realise just how much of an invasive police state we're in until we actually think about trying to avoid it.
I'm unsure of how severely couriers are regulated and what their obligations are to ensure they aren't smuggling something. USPS, UPS, DHL, etc frequently deliver narcotics and other contraband, so they obviously aren't expected to catch everything. Will look into it when I have time... would have to be done in a medium-large city first as proof of concept since this service would have a major business disadvantage of not being able to hand off packages to other courier services. The service could be "standardized," though... or franchised - whatever works to increase the size of the network.
Other regulation I was thinking of would be state or local regulations on how long any merchant must keep logs/receipts of transactions for police. Tax compliance might be another sticking point. Would probably suck to go into an audit and not even be able to use the blockchain or keep local records. ... Or maybe couriers don't require special licenses? Could probably fly under the radar for at least a couple years if so.