I tend to book fairly early, but it really pays to check in periodically. We are going to Kauai for 2 weeks arriving 2 days after Thanksgiving. DH prefers an intermediate size SUV. When I looked in June, Costco was about $1100, so I booked prepaid through Autoslash for $953 with Avis. You can cancel for refund up until 48 hours from pickup. I checked again mid-September, and Costco was down to $835 with Budget. So I booked via Costco, canceled Autoslash and got a refund.
I checked Costco again today, and prices have dropped significantly. $615 for Alamo. Booked Alamo and will cancel my Budget reservation.