I worked at the USPS for a short time delivering mail. Its highly unlikely that someone took the time out of their day to wreck havoc on your NRA magazine. The automated sorting system doesn't aggregate all of your personal mail through the same machine, that duty is spread out among like 50 sorting racks. Most likely your magazine along with other random magazines probably got sorted through a machine that was malfunctioning and it cut the cover clean off. In those machines the mail is moving at a good pace.
They deal with so much mail any given day that even taking the time out to cut your magazine cover would seem odd, as well as the postal inspectors and cameras watching from above in the sorting centers.
Also, saying that everyone is a zero at the post office is kind of rude. Seems like people only remember the times something fucks up and not the other 364 days. Like any government job you have people who care and those who don't care. I had the impression that postal workers were lazy and it was an easy job before I started. I don't feel that way after working there. Coming in at 6:30, sorting mail for 700-900 plus mail boxes in 2-3 hours, loading that truck with mail trays and the endless surge of amazon packages. Delivering that mail, multitasking, remembering small parcels as well as large boxes in delivery order, getting out in all weather to run to the door and deliver those packages. Being under a constant time crunch where 5 mins can ruin your whole day is beyond stressful. Working 9+ hours a day everyday but your one day off per week. They say you get a 30 min lunch, but no one takes one. Its a hard job, and one that everyone benefits from.