Do you have domestic surveillance problems? They are all through the postal service, to the point I think the Postal Service may have been created as some kind of domestic spying honeypot, where you give the agency your messages, they promise you they cannot be opened because it would be illegal, and in reality they have been opening and reading the mail of everyone important for the life of the agency.
Where I was when my domestic-surveillance/Secret Police adventures started, the local surveillance/gangstalking thing actually took over the back area of the post office. They posted foot surveillance all around it, and got real worked up if you drove behind the post office and sat there. All of a sudden there would be cars all over, and guys coming out the back of the post office to glare at you.
And that whole thing is set up like a public/private partnership run off the books, so if you piss off some big-wig in town who they recruited as an asset for something else, that big-wig can ask his handler to have you harassed, and next thing you know, you are getting followed and harassed by their army of mental-midget foot soldiers.
If you have anything like that going on, you cannot use the post office. Even Fed-Ex or UPS could be problematic as the big domestic spy network in America, encompassing stuff like DEA informants and so on will recruit through them to monitor package deliveries, and that network is available to the harassment people too. It only takes one person to locate and disappear your package for their handler. You would need to use a private courier or something more personal.