Pencil pushers and the economics of recycling

Reiter said the boxes are a cost to the Postal Service and it supplies them for free to customers who pay for priority or express mail. She said enforcing the no-recycling rule would keeps postal rates low for everyone.

So throwing them in the dumpster after you used them is fine, but if you re-use them, the USPS incurs a cost that raises postal rates for everyone.