How Should I Transport Cake?

Don’t set a cake on your seats. There’s potential for the cake to slide and tilt. The same applies for the front seat. Seats aren’t level and if you have to slam on your breaks, well, then your dashboard is going to be covered in buttercream.
Don’t put your cake in the trunk if it’s warm outside and your back seats don’t fold down to open up that area to allow for the A/C to keep it cool.

You DO have a better chance of having a level surface if you put the cake on the floorboards. Push your front seat all the way back. Add some non slip shelf liner (the rubber stuff that’s in a grid pattern) on the floor and set the cake box on that.
If your trunk opens up to the rest of the car, or it’s always cold in your environment, you can travel with them in the trunk.
Put your cake in the coolest part of your vehicle in winter and summer.

How Should I Store My Cake Once it Arrives at the Venue?

Refrigerate your cake once you arrive unless you are serving right away.
Remove cake from refrigerator an hour before the event to allow the icing to come to room temperature.
I recommend putting the leftovers back in the refrigerator after the event, in a covered container to maintain freshness.