I believe that for MasterCard, the credit limit must exceed $2000 for the rental car coverage option to be valid. I use a CapitalOne Platinum MasterCard for car rentals, and, like you, credit card coverage is primary for me.

I had a small dent caused by a stone hitting the roof of the car a few months ago. The rental agency submitted the claim directly to CapitalOne, who paid it in full except for "loss of use" and administrative fees. The car rental agency waived those charges, apparently, as I never heard anything more. Those fees were about $200, and the damage claim was about $600. Both amounts appeared excessive to me.