I would like to give you some feed-back on above three points:
Kundenservice: the booking was very easy and the information given came fast and was easy to read and understand. But when we arrived to the parking lot at the specified address, we were called by one from the company and was asked to drive to the airport instead of delivering the car at the specified address. At the airport we did not get a proper receipt and did not feel safe by leaving the car keys with a complete stranger and let him drive our car.
Parkplatz: When we arrived to the parking lot, we could drive directly into the area but there were no-one there and we did not need a code or anything to get in.
Flughafentransfer: It did not work as we were requested to drive to the airport. When we returned to the airport, we called Public Air Parking Shuttle to be picked up, and I was asked to call again when I was in the arrival hall. When I called from the arrival hall, I was informed that I had to wait 10 minutes as he had brought the wrong car to the airport. After 30 minutes I call again, and I was told to wait another 1 or 2 minutes. After further 10 minutes he arrived with our car.