Is the import levy described in Article 21(5) of the Decree [of the Flanders Region] of 23 January 1991 on protection of the environment against fertiliser pollution, which is imposed only on the importation from the other Member States of surpluses of manure derived both from livestock manure and from other manure, irrespective of whether these are further processed or marketed within the territory, and whereby the levy on those imported surpluses of manure is imposed on the importer, whereas in the case of surpluses of manure produced domestically the levy is imposed on the producer, to be regarded as a charge having equivalent effect to a customs duty on imports, within the terms of Article 30 TFEU, even though the Member State from which the surpluses of manure are exported itself provides for a reduction of the levy on the exportation of those surpluses of manure to other Member States?