Yes, you go to the website and enter in all of your info. Once you find a rate that suites you, then you make the reservation. you will recieve an email confirmation from the actual company that you are booking the car with. You take that with you at the time of pick up and that's it. it's as if you have booked with the rental car company all together. We have used this site four times in Florida. Twice in Orlando, once in Ft Lauderdale and once in Ft. Myers and we have never had a problem getting what we booked. We always book a minivan.
I can't vouch for Dollar, as i have always found a better rate with Thrifty so that is the only one I have ever used from this site. It's really not bad to rent from them. They are located right at the base of the Expressway, so you can jump right on there and head over to Disney.
Good luck & happy travels!