IAHt, good suggestion; I looked back at the booking, it was a 4 day maximum, so that didn't apply this particular time.

However, after turning in the car today, I went over to the Preferred booth to inquire. They looked at the modified reservation and the original reservation and couldn't figure out why the original ADD DAY rate hadn't been used either online or when I used the phone. They gave a refund of the difference and took off the $10 fee.