You come out ahead very quickly getting a credit card that will cover the rental car (e.g. a Chase Sapphire Preferred) and paying the annual fee instead of buying insurance through the rental car company.
I've never seen a no-annual-fee card that covers loss of use though. No-annual-fee card damage waivers are also typically secondary to your personal insurance (meaning you have to make a claim and your rates could go up), while the CSP's insurance is primary so your personal insurance is not involved at all.
