Do not understand the curious competence / incompetence of the post office

I've been mailing out a lot of books, both ARCs of mine and those of friends of mine, which means I've had to learn the cheapest and easiest ways of shipping things. And the winner, hands down, is the Post Office's Flat-Rate Priority Envelope. It's only 5 bucks, regardless of weight, and whatever you put into the envelope will get there in two days! Fur…