Trees were killed to make those boxes in the first place (do they have 100% recycled boxes yet?). Better to not order than to dump or even recycle the boxes.
Sure, but trees are not a good reason for that. I'll try to edit and add a better citation[0], but making cardboard boxes is more efficient than going into forests and cutting down trees.
[0]: http://www.internationalpaper.com/company/regions/north-amer...