If you’re a Costco member, use their travel site. The US fees are amazingly low. I’ve had a full-sized car in OC, for example, for $89/week all-in.
Most premium cards, like Amex Platinum, offer comprehensive coverage. In Europe, you usually need to get the insurance. I just saw an Audi A3 on Europcar in Austria for $100 for 4 full days with all insurance only $65 more.