I do believe there's some kind of package to package payment involved. Because I was dropping stuff at an office that is closer to me, but technically not my actual office that delivers my mail. One day when dropping our stuff there, the lady started giving me grief and asked if I could change the return zip, because they "weren't getting paid for my packages using that zip". I had no idea what she was talking about. I said: "I can't change the zip, I don't even think it will let me do that". Then I realized she meant the second zip, where you can put in a zip where it is actually physically starting it's journey... which in this case would be the office I drop at. Next time I came back, I asked if that's what she meant, and she said: "Yes, we don't get paid for delivering another office's packages". I was like... OK, whatever. So I started putting the zip for their office on their. As usual they didn't explain it well, but somehow, that's how it works. Which was news to me.