Dont forget some of the discount folks out there if booking in the USA (for non americans) too like 'happy tours USA' - sounds dodgy, but I have found amazing deals with them. Won't beat Hiddy's deal in the homeland, but for all in (additional driver chucked in too), you can choose which car company to use (their cheapest rates use alamo, but can use others if there is a preference for one that gives BA miles to bring slightly back on topic, including Avis, cant confirm though if it will help you with avis tiers), and at a third of the price of hsbc premier discounted hertz for a booking over xmas/new yr in california, I haven't beat it yet!