I haven't shipped any guns USPS but I know they have a theft problem. A couple years ago my wife sent a birthday card to a grandson. She put several $20 bills inside the card. When it got there, the envelope was cut open, retaped, and no money. Somebody was opening obvious card envelopes looking for cash or gift cards.

“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.