I had this problem during my last trip to Maui. I booked my car through hotwire to save money, but my flight times changed twice (United Air). They were fine with me picking the car up early, but only gave a 30 min grace period when returning. I asked about paying an extra couple of hours and they told me I'd have to pay a full day.

I kept checking hotwire too see if the deals were changing, they did change a bit, but not more than a few bucks a day. Might be safer to hold off on the car until you are sure the flights wont be changing or try one of the companies listed above.