Please see the list of items a rental car company can charge you for damage to the rental car. Some of these expenses may not be covered by your insurance. This is why I always take the rental car companies insurance.

Damages to the car: (covered by your insurance)
Appraisal fee: (covered by your insurance)
Towing and Storage: (limited coverage)
Administration Fee: (undefined)
Loss of Use: (if you have it)
Diminished Value: (undefined and can be substantial)