When using USAA, AA (and im sure others) and no awd #, Avis.com gives you the option of
Paying Now and saving of up to 30% off in addition to other discounts, although the best I've seen was 15%. I have some tips that I picked up after working with it for a while.
A: You only get the pay now option if you reserve more than 6 hours in advance.
B: If you have a coupon on the rental, you will receive a credit back from avis some time after the rental as when you reserve the car your cc gets charged the full amount and only after you return the car the system registers that you paid too much and you get it back as a credit.
C: If you need to cancel the reservation, if its 6 hours prior then you will get back the amount back less $25. If its within 6 hours you get charged a no show fee of $100. The $25 is usually easy to get off by calling cs and if its within 6 hours, I was thinking you could change the pickup time for later than 6 hours then cancel although I've never personally tried that.
D: It does get difficult to change something ( like awd) once you get the counter as its prepaid; although that would depend on the IQ level of the agent helping you.
E: This rule applies to any reservation: If you reserve using a preferred account, anytime during the day of rental the airport will "check you" into a car. Which basically means their assigning you a car. Once that happens, you cannot modify the reservation online as technically you checked out the car. I once reserved a car at the airport and I ended up renting from a local avis and the system would not allow the rep to rent me a car until she called and had the airport take me out of the car that they put me in there...