I am also getting the "Car Class Not Available". I was screwing around this morning trying to get a better deal on a convertible @ OGG, then suddenly the error came up.

Thought I had messed up the reservation, so I called in and they were able to confirm everything was correct.

Upon further looking at the website, it appears that a convertible is no longer available for the dates I am in OGG...wonder if that has something to do with it. In any event, they should fix their website. I let them know about it on the phone.