We have a daughter and her family who live 3000 miles away so I am always mailing packages. I also order many Christmas gifts, etc. and save the boxes to re-use. Removing the mailing labels has always been so annoying, but I happened to think of using my blow dryer to heat up the labels. Usually they peel right off! I was elated!
I reuse a ton of boxes for Ebay and I find the easiest thing to do is undo the box, turn it inside out and tape it back up, only takes a couple of minutes. It is well worth the effort to do this since some boxes have a bar code printed right on the cardboard and can cause problems with the barcode readers used by the post office.