Booking.com Overcharged $100 above the agreed price

Booked a hotel using this service. They simply handed the hotel my credit card details, allowed the hotel to charge their usual rate (not the rate that was advertised on booking.com nor the rate I received a confirmation for) and won't engage in any discussion over a refund. I am now $100 out of pocket. Don't bother using them. Their low rates are not genuine.