Avis not honoring web site booking, which was booked months in advance

I booked an Avis vehicle from South Africa, months ago already, at Munich Airport in Germany, using the Avis web site. When I got to Munich Airport on the 3rd of July, I was told that there were no vehicles available. Now note, I had booked MONTHS ago already, and even received a reminder via email. I was then **** to and told that I would wait half an hour. After an hour, I went back and another Avis manager told me that there were people who'd been waiting since 8am that morning (this was 16h00 in the afternoon.) There were many other customers with the same problem also waiting for cars. I was forced to rent a car from Sixt, at the same airport, and Sixt's agents told me that this happened every day at the location. It seems Avis is running some kind of a **** operation whereby they trick you into booking, knowing full well that they don't in fact have a vehicle for you. Never, ever, ever again will I use Avis after this shocking experience.