thanks for the tip.
I've just made a 10EUR reservation here:
PARKEN IN DÜSSELDORF - PARKOBJEKT FLUGHAFEN - BAHNHOF
It still is 10EUR/day.
You can pay +1.5 EUR for a refundable fee, but it doesn't worth it for me.
It saved me some change, as other options were starting from 14 EUR/day and official airport parking is even more expensive (50+).
Thanks again