Rented from Hertz at AUS about 10 times last year, and never had a problem with billing or lack of cars. The estimated total given by the Hertz US site has always matched the final billed total for me. Can't say I've experienced long lines; I'd suggest Hertz Gold Plus Rewards if you haven't already registered.

That said, many of the cars at this location are higher mileage and showing some wear. Every time I've gotten FCAR from AUS it's been the older generation fleet-only Impala (very 'classic' GM and a strong engine, for what it's worth) but I tend to arrive fairly late. I've seen more modern FCARs (Malibus and Sonatas mostly) on return so they do have them. National/Alamo and Avis/Budget do seem to have newer cars at AUS than Hertz.

They've started doing the upsell again even to Gold when you leave the lot but they're not very pushy about it. Never a problem to turn it down.