I'm really tired right now,
but ...
It looks like I could set up a weekly order for 5000 boxes, in 10 assorted sizes, and it would all be free.
In fact, after thinking about a PO Box Castle, I'm pretty sure that I'm going to build a couple of dog houses, and maybe a storage shed, out of a bunch of 'em. I can "tape" them together with free shipping labels!