Someone from Canada must have sent me a parcel and indicated a value.
When someone does that and it comes from outside Europe the PostNL aks for taxes for the amount that is indicated. You have to pay in advance, or else the parcel is returned. I missed the deadline for payment on this parcel, so it is returned. Not sure who sent me something from Canada. Also a tip is : indicate it is a gift with no value. Otherwise I have to pay taxes on the amount you indicate.