Some travel insurance plans offer coverage for rental car damage. This benefit only covers the costs of damage to the car, not any liability or property of others. Depending on the plan, this coverage may be included or an additional upgrade.
The cost of the rental car could potentially be included in the trip cost for trip cancellation and trip interruption. The rental car payments would have to be pre-paid and non-refundable in order to be covered. If there is no financial penalty from the rental car company for canceling the reservation, then there is no need to include that in the trip cost.
Always refer to the certificate of insurance for details.