Carrentals.com Nu Car Rentals Unprofessional, non-responsive, non-existent customer service

Utilized the website Carrentals.com to secure a car rental through Nu Car Rentals.  When I arrived at the counter Nu Car Rental associate had asked for a credit card to charge. I provided them the same credit card and immediately the associate had advised to me that my credit card was not working.  I was confused since I used the same card to secure the reservation but I called my bank nontheless to see if there were any issues with my credit card. They had advised that the card was in good order and that there wasn't any attempt at the card being used a few minutes prior.  I had asked the associate to try again and they had advised that it will not work and that they need to have a different card in order to process.  Once a new card was provided they came back to me and stated that the full-size car I had reserved was no longer available (within the span of 15 minutes they gave it away) and that I would have to pay triple the amount for a mini-van.  I declined and asked for cancellation.  Nu Car Rentals had advised that I had to contact carrentals.com to get my refund. To add, carrentals.com does not have a customer service line.  So I emailed stating my case to their support email address.  They respond by telling me that my request has been sent back to Nu Car Rentals.  It has now been a week and I have yet to get any resolution. This is borderline fraud as I did not utilize their service yet they have kept my money.