We suggest that you email your complaints to firstname.lastname@example.org. If this will not lead to a solution you can choose to report this to Stichting WebwinkelKeur. Since February 15th, consumers in the European Union have the option to report complaints to the ODR-platform of the European Commission. You can find the ODR-platform via http://ec.europa.eu/odr.
If your complaint is not threated somewhere else, you are free to use the platform of the European Union.