The dog food that I use comes in a bag that is printed on the outside and looks like very heavy aluminum foil on the inside. When the bags are empty, I cut them open so that I have one large flat piece, wipe them with soapy water and rinse them. There is no dog food smell.
Then, when I am going to mail something, I cut and fold them to whatever size I need. I fold one edge over the other (no staples needed) and seal with packing tape. A permanent felt pen will write on the surface and will not smear. They are virtually bomb-proof and much lighter (and therefore cheaper) than using a sturdy cardboard box.