Best Car Rental Service: Book Once, Get the Best Deal with AutoSlash

I just discovered Autoslash.com. I tried it a couple of weeks ago, booking a car rental for an upcoming trip. Two days after booking the best deal I found there, I received an email that Autoslash had found a better rate with the same company and had canceled and rebooked my car at the better rate. No work for me, just $34 savings.