We've seen this sort of thing before with Hertz unfortunately. The CDP you used on the reservation may have has the rate plan change which can cause a price change. Really nothing to do except to follow Hertz's advice and argue your case at the rental counter. It's a pain, but better than the alternative.
You might also just use the link below to track the rental to see if any lower rates are available. It's a long shot this close to the holiday honestly, but we do have some pretty deeply discounted prepaid rates, so worth a shot.