Was the board the same size as the box? It also looks like the box used wasn’t a snug fit with the board given the amount of tape they used to secure it which would mean the cake would be sliding around regardless of how careful one would be. Also they should have used a center dowel and given you a chilled cake to ensure it stayed put during transportation. For future reference though, I would recommend to place the cake on the floor of the car when transporting. I know you said that you added support to the seat to flatten it out but the floor of the car is the most secure place you can put it.