Booking.com/cars (Europcar) Ripped Me Off

I rented a car via booking.com/cars. Even though I PAID THE FULL PRICE IN ADVANCE, when I arrived at the pick-up point at the airport they told me I was too late and my car was not available. According to them, I was supposed to pick up the car at 11:30, but my flight arrived in the evening and the car was not available anymore. I had to pay more than double the agreed price again in order to get another car. I find this totally unacceptable: when paying for a service in advance, that service should be unconditionally available to you. It was not a simple reservation, the full price was already paid. Booking.com refuses to refund the additional amount I had to pay, and the customer support is terrible: communication is over email, and every time a different person replies (so far about 10).