I just rented a car for a trip. At first I tried the various discount/lowcost travel sites. Then I went on one of my cr card sites. They love to sell you parts cars travel! I couldn't believe the rates. I booked a car, even paying with another card. When I do something like this online, I take a screenshot of every page I go thru in the process. Take a printout of them with you. This way there are no surprises and you have proof of your agreement. (I was esp suspect of this one, but absolutely no flags came up throughout the process)
Great advice from LY; check your coverage and do not pay them for gas. Gas up before you head to the airport.
Have a great trip!