Just got home from USPS. Guy behind the counter refused to accept my pre-printed DC label. I was shipping a hardcover in a bubble mailer. I printed the single-page "wrapper" with DC as I have dozens of times in the past. When affixing the address label to the mailer, I cut around as much as possible, including the upper right corner where the stamp box is. USPS refused to accept. I asked for a supervisor, logged on to PBS and showed her what I'd done. She reprinted the shipping label, and let me mail the book. I had no idea that little square had to be there, but apparently this USPS on this particular day says it does!!

The first time I mailed using the printable DC I left the little stamp box on and it confused the postal workers at my post office.  I've been cutting the stamp box off ever since. There's absolutely no reason in the world for that box to remain there.

My guess is that they were confused about whether or not you'd paid postage online because that's where the postage box usually is. There are very few DC-only printing companies so I'd bet they were looking for a postage paid box not a postage due box.