I have successfully shipped matted prints in styrofoam that is about 2" past the size of the matted photo's. See Les's post. I put the photo and mat in a clear plastic bag and tape it to one of the pieces and then place the other side on the pile and tape the hell out of it. This stuff I have used it sold in Home Depot and is wall insulation. It weighs nothing and has a silver paper covering. Comes in 4 foot x 10 foot panels. A little time with a knife and you are all set. You could add a piece of Masonite for stiffness if you thought you needed some extra.

lee\c