Substitute "masonite" for "cardboard" in ROL's post above and you will have a nearly indestructible package. I also put a second bag over the print which gets taped to the masonite. The only time I have heard of a problem of all the people who I know who do this is when someone put a forklift through the print stack. Otherwise, you are pretty much guaranteed to have the print arrive in pristine condition. Cardboard can bend even if you double it up so it is best to avoid it. The masonite looks a lot more professional too. Glue a label on it and off you go.

Just go to Lowe's or Home Depot and they will even cut it to size for you. It is cheap.