Rentalcars.com is a complete rip-off. I booked an Avis rental car through them for a Costa Rica trip. I followed all the requirements including having a letter from my bank confirming that I had rental car insurance coverage with my credit card. When I arrived, Avis wouldn't accept the letter saying that since I booked through a 3rd party, I had to pay for their coverage (an additional $152 for the week).
The rental was fine and I got it dropped off; Avis handed me a receipt and said everything was fine. When I got home, I went through my bank statement and saw that Avis had also charged me for the car ( I was charged by Rentalcars.com when I booked online). I corresponded with Rentalcars.com about both the over-charge and the double charge. They said they could help with insurance issue, because I signed the Avis rental agreement (what else was I supposed to do?). It's now been over 2 months and they won't address the double charge. I had to file a dispute with my bank. I'll never use these guys again.