I'm visiting my folks and my mother just received a shipment of boxes that she ordered from the USPS. She has been selling items on eBay and needed some boxes to ship the items. Turns out she was able to order the boxes from the USPS for free with free delivery. The same boxes that you pay for at the post office you can get for free. Here is the link.
Update: Just noticed that the link does not work. To find the items just search for supplies.