Out of curiosity, what do you consider the advantage of using "prepay and check in online" rather than registering for Alamo Insiders, National Emerald Club, Hertz Number One Gold, or something similar?
Before this thread, I never realized that some people have to check in online and prepay before they can get their car.
I just make reservations with National, often many months in advance. After I arrive at my destination airport, I just get in a car (from the aisle that matches my reservation) and present my drivers license at the exit gate. My credit card is not billed until I return the car.
No need to check in online. No need to prepay.