who ever told you not to use the usps is a fool and telling you to use a private company instead was the worst advice you could get honestly.. if you go into a ups or fedex store, they will tell you right in the store, they have signs hanging all over the place, that they can open your package at any time they feel the need for it.. they can and will do this if they want to..
now, the post office, the only people who are going to open your shit is customs if it's coming from out of the country, or the post master general.. and even he/ she can't just open up any package they want to, they need to have some sort of reason to open it up.. stinky ass pot smell coming from a package, dripping liquid on the outside of the package, ticking noises, etc.. other than that, no one inside of the usps is allowed to open any packages that go through the post office...
like i said, walk into a fedex, ups store and they openly admit that they do and can open packages whenever the feeling strikes them, for ANY reasons... it doesn't have to stink like bud, it doesn't have to be dripping stinky chemicals out of it or anything.. they can just simply open your shit for what ever reasons they want..
stick with the post office.