I believe I understand your complaint perfectly.

The impression I got from your original post is that you looked at prices for CPMR and saw an attractive price for PTMR and put down in the comments you will be driving to Poland. At a later point in time, you decided to change the return and called Sixt. Sixt agent then read your comments about driving in Poland and informed you that PTMR can not be driven to Poland and at this point in time, the cheapest vehicle that can be driven to Poland is an ECMR which is more expensive than the initial price you saw for CPMR.

Your complaint is that had the website informed you when you initially made the booking, that a PTMR can not be driven to Poland, you would have stuck with CPMR and thus you are upset that "that the system allows booking that is against the company policy." The point I'm trying to make is that this issue is not unique to Sixt. No car rental website can understand comments made in the comments box nor has any major car rental company design their website as you suggested. You would have experience the same issue with Hertz, Avis, Europcar had you selected a car class that is prohibited from going to Poland.

As for printing out the conrtract in English, I agree with you. I've found customer service from front-line employees in Sixt Germany (Dusseldorf) to be subpar to Hertz Germany or even Sixt USA. They just don't seem to be as empowered to have any flexibility in ensuring customer satisfaction.