SO you are a seller. And you need to ship in  pre-used boxes rather than buy new ones, to keep costs down. There are several methods, but this one seems to add the least weight to  the box (virtually none). Well, here's how you can obliterate the old printing on it, and make a nicely decorated box. AND add a custom look to your shipping!
Easy:
Two cans of that spray paint you have around the house, and a piece of cardstock or paper is all you need-- besides a good sturdy box.



DO THIS A FEW DAYS AHEAD OF SHIPPING TIME.
1. Take your box outside or in a garage etc. (ventilation).Take a dark color, spray over the printing on the box to make sure it's obscured. Let it dry. Might need 2 coats.
2. Cut a simple stencil from the cardstock, I used a box cutter.
3. When first color is dry, hold the stencil onto the painted area, and spray with the lighter color. A light mottled spray is fine, if you want.  Looks great! And obscures the printing for the post office's requirements.
4. Let this dry and cure for at least 24 hours, longer is better, to minimize any paint smell.
ET VOILA! A nicely decorated, reused box.

Below: I used dark brown, and ivory on this one.



GOOD LUCK!!! HAVE FUN!!!