Walmart sells small boxes for about $1 each. I use them when I mail things to Elen, USMC.
I have those, thinking they would be great for books, but they’re too big for me to handle if they’re full. I can use them for something else, but I have to have book boxes. U-Haul will buy them back at full price when I’m done with them.
Those small ones are 14” X 14” and that’s far too much. Book boxes are about 12.5” X 12.5”.