Image "my rental car" by David Lee King is licensed under CC BY-NC-SA 2.0.
Recently, I discovered Autoslash.com and put it to the test. I was impressed! We not only found a rental that was cheaper than what I had found on my other favorite sites, but it also undercut the price of a Costco rental. I'm not disputing that Costco can often find low-priced rentals, but because they search only a few major sites, they don't have the breadth of Autoslash.com.
What's more, even if you have secured a rental through Costco, you can still enter it on the Autoslash site and have it tracked. If Autoslash finds a lower rate, it will let you know. I've only used it once, so time will tell, but I'm expecting Autoslash.com to win out over the long term. If anything changes, I'll keep you posted.