You should reserve the car you want with somebody so you have it for the dates you want.

Then you can start bidding on Priceline. You have a good amount of time till December, so start at half what your reservation is for and increase it a dollar or two each day when you rebid if it wasn't accepted.

Set your upper priceline limit, make sure you are comparing all the fees they add in and not just the daily price, and understand once your bid is accepted you will be charged, and can not usually change the reservation.

Make sure to enter your parents name and ages for the rental and not yours.

Convertibles are always in demand in Hawaii, you may find that Priceline is not really going to save you any money which is why you reserved a convertible with a regular reservation before starting.

Cancel that one if you get the priceline one.