This is complex - I have done some tests myself. The easiest thing is to make a cake that stays fresh for a while (fruit cake, some chocolate cakes) and cover it in fondant. Nix the buttercream.
Now if you want to use things like buttercream and yellow cakes, you need to embark on a mission. The mission is: Make a cake box and insulate it with aluminum foil. Fasten the cake as best you can - no wiggle room for the cake. pack that box in a box of styrafoam peanuts. Now THAT box must be put inside a third box (preferably made of styrafoam, like a strafoam cooler almost) which contains dry ice (dont touch it though). Talk to your local UPS store to discuss the fact that you are shipping a cake and the box can't be turned upside down, etc.
Hope this helps
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