If you plan to ever be in a situation where you will need to know what the strongest legal/letter size boxes are, bar none, it is this one:
About four and a half dollars per box, but these things are double walled, double floored, and have deep lids. These can be stacked several boxes high and when you come back in a few months they won’t have collapsed under their own weight like those white boxes you find in Staples do (sad but true, those white boxes at Staples usually don’t even survive being carried from one room to another). Granted, who can afford $4.50 per box? But bookmark it just in case.