You have to prepay when you make your reservation, not later. You have a choice of either prepaying or using a code. Sometimes it's cheaper, sometimes not. You would have to make a NEW reservation and put PREPAY in the contract code box (or click on the prepay link), which means you can stack the discounts. You can add a coupon. I used the prepaid for my February 2006 and April 2006 reservation since it was cheaper both times. Using the prepay methods has its ups and downs. Often you get the best rate doing the prepay methods, BUT, if the rate changes and goes down, you have to cancel your reservation which has a $5 fee, which becomes a $10 fee if you get too close to the dates. It's worked for me both times, but I rent alot and am pretty good at knowing the difference between a good deal and excellent deal. (I had an excellent deal in April, renting a mid-size SUV for $90 a week before applying the coupon.) This is probably not the answer you wanted.