This question has been asked in this sub more than once and the advise I give on the topic seems to satisfy those affected. I wrote this post in hopes that it might help others and so that others can appreciate this idea and gain some personal insight of their own.
Feeling like your personal mail is being fondled and molested by nefarious, tax paid pervs? Heres a solution!
Mail yourself something (preferably from a post office as far away from you as possible).
Package it carefully yourself. Remember exactly how you placed things. Take pics if it'll help.
Get creative. Use colorful tape, make shapes over the openings of the package with it. Use a specific number of packaging peanuts that you counted out. Wrap the object you mail in some thin holiday wrapping paper. Tape that too. Go crazy! It doesn't matter if it looks sketch, shucks, might be better for it. Hell, maybe even hand write the info on the pack.
The item you send doesn't matter so long as it's legal (I'd send one of those motion sensor cameras that hunters use to capture night time wildlife). Remember, we're trying to find out if our mails being tampered with.
Conduct this experiment as many times as necessary. Paranoia is a mutha-fukka.
I hope this helps anyone out there who has ever had this issue (we've all been there).
Mail myself a kilo of flour