Kal, and everyone - to ship safely (I've always had good luck like this):
1. Make sure the print is in something water proof, like a print file sleeve that's been taped up.
2. Use fairly heavy cardboard, four pieces, slightly larger than the print.
3. You know how the cardboard has a structure, you can see its patters, and it bends easier one way than the other. Criss-cross that pattern with two sheets on top and two sheets on bottom.
4. Tape the print to one of the cardboard couples, lay the other cardboard couple on top, and then tape those two together - all along the edge.
This makes for a very stiff and secure envelope. I haven't had a single print, even matted to 16x20" become warped this way.
Or, if you're sending it my way, put it in a sleeve, and send it in any book about/by Mary Ellen Mark, preferably signed, and send it over...