Most decent credit cards come with at least a CDW (collision damage waiver). The JetBlue Amex has damage and theft/loss rental car insurance. You don't need to buy that insurance from the rental car company.
5. Car Rental Loss and Damage Insurance is underwritten by AMEX Assurance Company, Administrative Office, Green Bay, WI. Coverage is determined by the terms, conditions, and exclusions of Policy AX0925 and is subject to change with notice. This document does not supplement or replace the Policy.
It's always a good idea to call the credit card company before the first time you rent a car on a card to make sure you understand the terms and conditions of the insurance (is there a deductible? for example).