I am taking a last minute trip to Kauai next week to attend a funeral and shopped for rental cars today. Boy what a shock when I saw the rates. I went on Priceline and Costco, Orbitz and Hawaiian Air's site. I finally booked Costco(that was the best rate of all) just to have something and it was $373 for a compact for 5 days. I put a request in with Discount Hawaii Rental Cars and just got a confirmation back and the same exact car for the same exact dates and times and even company (Alamo). Through this site is was $178.02 including taxes etc.
So my point again is shop around. Book something but keep looking.
Cancel any old reservations but print everything and bring it with you in case there's a mixup. That money I am saving is a nice meal at the new Poipu Tapa's restaurant.