I've shipped a small heart shape dense cake that was covered in ganache as testing to my family. I sent one from Ft. Lauderdale to Orlando and another cake from Ft. Lauderdale to North Carolina using USPS.
I wrapped the cake with clear wrap many times and placed in a cake box, then wrap the cake box, place the cake box in a bigger box. I marked the box with big letters (multiple times) "This Side UP" with an arrow. When the cake has arrived, it was obvious that the box was turned upside down many times. So I can't trust the "This side UP" sign and expect people to follow the instruction.
I have only shipped one cake: to my son in college. It was decorated in fondant; I wrapped carefully with saran wrap, then placed it in a regular cake boxed. I went to FedEx and they added shipping peanuts inside that box. Put peanuts at the bottom of a shipping box; carefully placed my cake box in it and added more peanuts until it was snug. I had it at FedEx Thursday morning, at 8:00 and my son opened his cake on Friday, at 11:30 am, on the day of his birthday. There wasn't one single dent on it! We did not use dry ice, or anything else, and it is my understanding that he and his friends were still eating cake the following Tuesday!
Cost: almost $ 90.00 after packaging, materials and shipping.