What does it matter if the box contains 5 pounds of books or 5 pounds of clothes, or 3 pounds of books and 2 pounds of clothes, as long as I'm willing to wait on slow delivery? It's the same weight, dimensions, and amount of trouble the post office has to go to to get it to it's destination, so what difference is it to them? It seems extremely arbitrary and petty, and designed to milk people for extra postage money.

This was incorporated into postal rates donkeys' years ago, designed to promote literacy, education, and an informed public. Books, etc, were "educational materials" and education was prized. Thus, they allowed books and educational materials a slower but cheaper rate to ensure the poor and lower classes could still obtain educational materials.

Another of the original arguments was the Bill of Rights -- Freedom of the Press. So again, there was momentum to provide a cheaper freight rate for books and printed matter, like newspapers & other educational materials.

This has morphed now into "media rate." It was not "arbitrary and petty." It had very specific purposes in our history.