I'd go to my local True Value hardware store or whatever hardware store you actually buy from and ask them if you can come in on "inventory day" and pick through the leftover boxes they've emptied that morning and when is the best time to come. They will have all sizes and shapes of boxes available. A university bookstore is even better, if you can intercept before the dumpster.
You'll need a big WIDE black marking pen to black out any redundant content labels that the PO won't ship.
I've been shipping used books for the last ten years and haven't paid one cent for packaging material, except tape....
I'm also retired ....and CHEAP. "Close," as my mother's clan would say. A complement in my family.
