I was just going to ask that doug. Did you see a picture of the damaged product? Did she bring it back to you to see? (i mean she was only 15 minutes away) If you doweled it like all your others and they have been delivered/picked up fine then I am wondering what exactly happened.
I personnaly would have done the same as you, offered to repair what I could. However you spoke with her about the traveling with it, and how it was doweled and you placed it in the vehicle. She was happy with it when she left, she paid for the services you provided, she was happy with your services, it is not your fault for her driving.
If you bought a car, drove 10 miles away from the delearship and got in a wreck and totaled the car, would you expect the dealership to refund any money? No because you paid for a product that was to your satisfaction, and it wasn't their fault you got into a wreck.
If they bring it up I would do as others stated. Explain how traveling with a cake takes a little more care than normal because they are not as stable.
Hope this all works out for you. I am sure if they were really happy with the cake then you still have a shot at the wedding cake--and wouldn't you be delivering it anyways