The boxes not only need to be reinforced, but, the contents need to be surrounded with enough bubble-wrap/etc such that the contents do not burst-through the sides/ends of the box when the shippers throw the box around.
I received a Silver Book in a Priority Mail box - the book had flimsy plastic around it. The end of the box was spread open, and the binder on the book was broken - because it moved rather easily inside the box.
Other Silver Book shippers much more carefully packed in the same box - no damage on the book inside, but a little crumpling of the box on the outside.
