Just happened to me today. Prepaid rate was much lower so I booked it. Rental agreement shows that I still have to pay around $30 more and shows a "Voucher" applied to the total for the amount I prepaid. Vehicle class was the same one I reserved (L) and there were no changes in the rental period. Can't waste my time complaining at the location (DCA) because I've had enough with them for the past few months and don't want them to tag me as a serial complainer, so sent a message through Twitter with no positive answer other than "they referred it to the right team".