Since you live in the states MR COMICBOOK, go to usps com and order some priority mail boxes for cardboard. You can order up to 500 at one time, and they are FREE. No-cost shipping, too. I accidentally ordered some FPO boxes, and I use those for cardboard sandwiches.
Yep, you get an awesome service through USPS. Royal Mail pails in comparison.
Using the Priority Mail boxes for other purposes (or just for the cardboard) will eventually increase the price for the rest of us. They're already spending additional money printing on the inside of the boxes which I'm sure they will pass on to the customer.
(I realize this practice also precludes re-using OLD priority boxes... but using NEW boxes for cardboard sandwiches is really theft.)
I buy 200lb cardboard from Uline. It costs money, but I'm assuming the person I send it to reuses it and sends it to someone else in turn. I also recycle old boxes and large cardboard computer boxes for these purposes.
Great point made here by Shin!
I cut up all the boxes I receive, and save all the packing material. I also buy diapers for my son from costco and that box nets me 4 oversized pieces for mailers. And any other boxes I can chop up.
Taking the new boxes from usps to cut up for shipping is going to catch up to all of us as shin said. The boxes are "free" but they are not really free. In shipping and stamp costs, we pay for everything.