It sucks to say, but your local post office personnel aren't above being scumbags. Here's something that happened to me, not Magic related.
I lived in a city neighborhood that required residential parking permits to park on the street all day without getting a ticket. Since parking near my neighborhood, in a garage, can run you somewhere in the ballpark of $1500-2000 a year, obtaining one of those parking permits can be worth a lot of money. I'm sure you could easily sell one for $500 cash, with no effort what so ever.
Anyway, each year we received notification from the city which was a form to return with a copy of our registration, showing that we still lived there. So, one year, we get the forms and I fill them out for both the wife and myself and send them in. A couple weeks later I grab the stack of mail that was shoved through my front door mail slot and there are two envelopes, from the Department of Finance (the office that issues the parking permits), that are torn open and empty. Not slightly ripped and maybe the contents fell out... like, someone took their finger and opened the letter end to end. To be clear, the rest of the mail was completely unscathed in any way. It was just the two letters from the Department of Finance that were manged and empty.
So I called the city office, explained what happened and the lady on the phone says, "OK, I'll void those stickers and send you out two more. Expect them in 2-3 days." Well, 2-3 days goes by and I pickup the mail at the front door, only to see two more letters from the Department of Finance which had been ripped open, emptied and taped shut. Again, no other mail had met the same fate, just those letters.
I took all four envelopes to the city Post Office, showed them to the Postmaster and he assured me that no one would ever do that and that my mail slot must have mangled those envelopes (but no others). When I asked him if my nefarious mail slot also felt bad and taped up the two most recent letters, he just scowled at me and said, "maybe, but it wasn't us!"