Yes there is a catch.. most cc will only cover only after you have exhausted your car insurance.. and yes you should get coverage from the rental agent.
why? beacuse when you get into an accident or if the vehicle is stolen they cover the car 100% no deductibles to you,
even if the car was vadilized in the parking lot.
tropic is fabulous but only rents Yaris, also recommend very highly More4less if Tropic has no cars available. We are on island now for 5 weeks and have a car from More4Less--great car and great service and he picked us up at airport and took us to his close by rental shop then we were on our way--very highly recommend them and Tropic!