I bundle my volumes between cardboard and seal them in plastic bags--they feel and look like coverless books from the edge (which in a manner of speaking they are), and that's all that's visible through the bag.
Then I pack and seal up the box, write BOOKS in several places on the outside.
I address it to a book bindery.
Then I thump it down on the counter at the Post Office and say "Media."
When the Postal clerk behind the counter starts to ask the question they always ask about hazardous materials or breakables, I just smile and interrupt with "Nope, just books."

Never had a problem.

Ah, the meek. Playing the long game.
Sneaky bastards.

--Denise Mina, Field of Blood