It's easy to book rental cars online. Payment is often not required to hold a reservation. Problem is you might get one too many.
Let's say you book Avis online, then call Gemut.com for a quote and find they have a lower price, also with Avis. You book the car with Gemut but, because there was no payment involved with the first booking, you forget all about it. Now, at the rental counter the agent uses your name to call up your booking in his reservation system. Maybe he sees two bookings, maybe he just sees the first, higher-priced booking, and that is the one he uses to print your contract. In your jet-lagged state, you sign the documents that are put in front of you without giving them much scrutiny. When you return to the U.S. you get a nasty surprise, you paid the higher rate.
Of course, Gemut.com will fully refund your second, unused, prepaid booking but you're stuck with a rental that cost more than you planned. Avoid this by canceling all bookings except the one you want, and make sure the rental agent is handed a copy of the voucher you were sent by Gemut.com. (Get an email quote on a European car rental, or, if you prefer, phone Andy at 800-521-6722 x 3).