1. Did you enter it as a PC with another discount code - Hertz has a sale coming up - or as a CDP? If it was a CDP, that is someone else's corporate rate and you can't use it anyway, but it's no surprise that the rate is higher. You need to enter it as a PC.

2. There is an extremely high chance of being upgraded from a 'standard convertible' group, not the assigned Mustang group, so use that.

3. If you have managed to get a Vette for less than a discounted standard convertible, you've found a bargain and there is really no more to say!