i would never have allowed them to take the cake.
i would have said that i had given them instructions in advance, to have a van or suv or flat area, and that the vehicle and circumstances that they present (small sedan, toddler in the back) is not conducive to delivering a cake. i would have told them that i or my husband will have to deliver it, given that they did not follow directions, and that we would have to charge them for delivery. i would have said that there is no way that cake will make it to the reception in one piece, especially if it had to sit in the car while the planner went home to change, and that i wasnt about to have my reputation and work ruined. if they didnt like it, too bad!
i like the idea of another poster, to have it in the contract that if suitable transportation is not provided, you would have to deliver it yourself and charge for delivery. or that anything over one or 2 tiers would have to be delivered.
sorry this happened. i know how much i sweat as i'm driving a cake to a destination, careful over bumps and potholes, etc.
i bought a roll of that rubber non-skid shelf liner at walmart, that looks like a waffle grid. i always keep a piece of it in my car, i use pieces between the cake plate and a box or large cake tin that i carry a finished cake in. i have cut off pieces of it and given it to people who pick up cakes (not too often, i usually deliver myself) so they can put it down in their car so the cake wont slide. this stuff is amazing, nothing slides on it. i like it so much, i lined the back of my suv with it. when i go grocery shopping, the plastic bags full of groceries dont slide around the back of my car anymore.