AutoSlash is a free (I love free) service for saving money on car rentals. You just enter the pickup and dropoff details, credit cards (to find their deals), and memberships (e.g. AAA) you have and you'll get emailed the best deals. There's is an option to enter an existing reservation, as I did, and you'll get notified when it finds better deals, like the one in the screenshot below. Rental cars are currently crazy expensive, so saving almost $50 was a welcome treat.