I hear ya. If we did things as we did in the old days, just use postal mail for everything we do, you’d be in much better shape. But as you suggested, even that is probably not totally secure from someone tampering with.
I wouldnt doubt the post office on privacysimply because any pkg tou send has its restrictions and those are selectively xrayed or pulled aside because. it looks suspicious and there are post office guidelines for suspicious pkgs.
Letter mail? watch a few ytubes on how letter mail is handled and you will see that its nearly impossible to tamper with.
There are dishonest postal carriers and mail handlers but they are not prolific and the post office deals with them swiftly.