Rental car insurance is a scam everywhere but perhaps worst in the States. The last thing you want to do is to buy insurance at the time of pick-up. Some hire car companies offer insurance packages at time of booking (which is better), but I've found it easiest to go through a broker such as https://www.discountusacarrental.com/. Alternatively arrange your own insurance independently in advance; eg www.insuremyrentalcar.com. (I've used other ones in Europe eg https://www.worldwideinsure.com/car-hire-insurance.htm and I think most of them will offer cover anywhere...)