Honestly, for what you're doing, get yourself a cheap ceramic knife. They come out of the box sharp enough to cut through cardboard and the best part is they never dull. You get yourself something around $20 - $30 and you will chew through cardboard like it's nothing and save yourself the grief of having to re-sharpen your knife or buy new blades for your box cutter. Don't believe me? Ask what Aldo from the NJ Steel Baron what him and his sons use.