FWIW, my last rental with Avis via Costco Travel was so cheap, I kept the car another week. Rental price seems to be just a little more than owning, which makes it a good alternative to a second vehicle.For cashback, I seem to get the best cashback by starting at cashback monitor and comparing all the options available. For absolute best cost, I recommend using Costco Travel if you are a member, or Kayak if not. Then use Autoslash to track the rental, they will let you know if they find a lower price then you can rebook.
Just as a point of info - I like autoslash too, but its terrible for international travel. Maybe it was just my locations Portugal, one-way, to Spain. kayak.com was the best I've found. Any other suggestions?I hope everyone is using Autoslash.com. I've saved substantial money by doing so. They got so much positive press that some of the car rental companies no longer allowed them to show their rates. However, you can input the rate you found on their site and they will usually be able to better it by the time of your rental.