Through QuickRent or Emerald Isle, you just walk out to the cars, pick one in your category and drive to the exit of the garage.
There is a booth with an attendant at the exit where you show your confirmation and your credit card. That is where they print your paperwork.
Yes, one day rentals are sometimes less than towncars, and it makes it convenient for stopping at the store, or going to another park on your arrival day. In my case as a solo person, it was less than a towncar, and slightly more than Mears. Plus I'll be able to go to Wal-Mart. Most towncar services prefer to stop at Publix or Goodings because Wal-Mart is out of the way.
For example when I arrive 1:30 ish, I will get my car from the airport, then go straight to Universal and spend the afternoon there. When Universal closes at 6pm, I will drop by Wal-Mart, and maybe even Belz Outlet, then check into my resort at WDW.
The next morning early, I will drop my car at the Dolphin, with the valet, then walk over to MGM. If I have a good feeling about them turning in the keys to the desk, I won't return. If I don't have a good feeling, I will wander back over to the Dolphin about lunchtime and doublecheck that it was turned in on time.
The day before I leave, I will be at Epcot that day. I will walk over to the Dolphin before the desk closes at 4pm and get my paperwork on my car, then I'll probably have dinner somewhere, maybe even Blue Zoo at the Dolphin. I will then go back over to Epcot. When Illuminations is over, I will pick up the car from the valet, and will drive my car to my resort.
The next morning, I will check out of my resort and head over to Universal for the day. I will go to the airport from Universal and turn in my car.
If the online system indicates that you can rent a minivan at the Dolphin, they should have one available for you as long as you show up at the time you indicated you would.