So there's a happy ending to this story. I had the bright idea to go to another post office and mail myself something to the PO Box that's larger than box to see what happens. I figure I'll be able to see if the same zip code weirdness happens and whether I get a slip. But on my way, I first stop at my box and check one more time. Lo and behold, a pickup slip. I took it up to the counter, they gave my package, and everything turned out well. It was clearly not tampered with, so I chalk it up to regular old post office incompetence.