A credit card wouldn't cover liability in any case, so best suggestion is to look for an all inclusive rate so you know what you will pay up front. A google search for USA car rental should come up with different agencies mostly located in the UK. Prices seem to be about double what you've been quoted, but include the insurance. Try different countries of residence since the rates vary, just make sure they include insurance. Also try the Avis/Hertz forums with their discount codes, which might reduce the rate significantly.