If you decline the car rental company's insurance and charge the rental fee to your Amex, then Amex has a free car rental insurance plan. It's essentially secondary to any other coverage you carry, so if you wreck a rental car and already carry collision insurance, then it hits your coverage first and the Amex plan just pays your deductible.
They've also got an opt-in plan where any time you rent a car with your card they charge you a $25 flat free, and then that buys you coverage that runs primary so you don't have to claim against your own car insurance at all.