This doesn't change the crux of your comment, but the post office does give them a price break; in fact it gives everyone a price break, all you have to do is sort your mail by zip code.

The way it works is if you send enough mail to the same zip code, then you pay a certain discount (a few cents per mail piece) if you package all of that mail together. The amount of the discount varies depending on if you sort by 5 digit zip code, or just 3 digit zip code prefix.