I usually just buy boxes and shipping supplies from Uline.com
They have a massive variety and box sizes.
Also to me in NJ I always get it the next day after ordering.
USPS Priority Mail gives free boxes - but there is a disclaimer on the box that says to be used for USPS only...
Besides that its "stealing" I personally don't think it looks professional to ship with a different service and USPS boxes
For international Fedex or DHL discounted prices PM me.
I also get discounted Fedex Express shipping, and somewhat discounted on ground. (Its hard to get big discounts, as the prices are already pretty low)