What is the best way to package a cake to ship it across the country?
id say no matter what you do, its not likely to work so well. all it takes is to be turned at an angle, and its slid against the wall of the box - you cant pack it in or that would damage the icing.
maybe do a test run before posting the real cake - what is it for?
I know Mike's Amazing Cakes in Seattle ship theirs (or at least they use to), you might email them and ask what their method is.