I used Expedia this last trip for flight and all inclusive car. The only "extra" paymeny I had was for the tank of that they provide ($35). I had the option to return the tank full for no charge, unfortunately I couldn't find a gas station on my route from South Beach and couldn't be bothered to drive around and find one. Had I put $20 worth of petrol that was required to fill the tank up that charge would have been waived.
I did leave a credit card imprint to cover any "incidentals" but, in the end, the petrol was all that was put on it.