USPS needs a warrant to look in the mail.....much safer.
Fed Ex, and the others can look if they want without a warrant since you are basically giving them that right (in the small print) when you ship it with them.
think your fourth amendment rights (a) mean dick shit and (b) can be safely hidden behind?, think again. The usps needs a warrant, which they will get because they will bring the box that "smells like marijuana" to a police officer who will certify that it is suspicious, receive the warrant from a magistrate, and proceed to search the box, fucking you over in the process.
FedEx and UPS could more easily search your package, since they're private and you agree to their terms, but sending shit overnight (or if you're super eager, same day), well packaged, and paid for in full, greatly reduces your chances of being picked up. They don't want to hold up your package because they want to fulfill their end of the agreement and keep you as a customer. They also don't want the headache of reporting their findings to the gov't and dealing with the ensuing case that fucks you over, particularly for small amounts--a couple ounces is way more trouble than it's worth to do all the paperwork etc. of getting you fucked. NOW that all being said, you are trusting UPS/FedEx drivers not to be curious, sniff your package, find out what's inside olfactorily, snake the goods from you and know that you're not going to come back on them. This has happened to me before. I've received a package from a buddy that was supposed to be overnighted. it got delayed for no reason while i was tracking it, and when it did arrive, a day late, all that the dude left on the front door step was the original FedEx mailer, re-taped shut.
Now, do I think this is a good idea?? absolutely not. particularly right now when this is the prime thing to be looking for if you are USPS/UPS/FedEx in WA or CO. Safer and more fun to just grow your own!
be easy,