Stopped for refueling and lunch. The GA Terminal is a very nice building run by Jet Aviation that also does handling. The people are friendly and helpful.
Handling is not mandatory, and we did not order. But we still paid €65 for "Safety cones GA apron", €25 for "mandatory infrastructure charges", and worst about €100 euros for noise fees to the airport. They charge both landing and starting fees and noise fees for both landing and starting. DA42 is in a higher noise category according to the airport fees schedule. Most SEP are free. No parking fee less than 3 hours. Our bill was at €250, which was hardly worth it. But if we had ordered handling it would have been another €300 according to the quote we got in advance.
Paid for fuel with Shell carnet.
Also - being IFR we had to organize a slot time. This was easy via phone to AIS-C. If you are visiting Duesseldorf and staying some days the bill might be justifiable, but we will plan a different fuel stop on our return journey. That said, it was relatively painless, and coming from a non-EU airport we needed an airport with customs.
Clearance asked me to contact my ground handler to put in a TOBT time in the system when I got my clearance, but when I informed him that I did not have a ground handler, that was not a problem.