Spanish Customs and Correos
One official told me to fill in a form online which was impossible both for me and for him and when trying to get in touch over telephone, it was impossible.
Julia at Yacht Port Cartagena spent many hours trying to help me out and more forms were filled in and sent (alltogether 6 different forms ) allthough without success. After 4 weeks with customs in Madrid the only choice was to either have them sent back to the supplier in the U.S or pay an agent to "release" them from customs.
The agent took 168 Euros to get the filters out (almost the same price as the filters including transportation from the U.S) and today they finally arrived to the marina,after 4 weeks and 2 days in Madrid.
Having sailed in many different parts of the world I have never experienced such a complicated customs/importation procedure.
As far as I understand, many other cruisers also experience problems in Spain when importing parts from outside the EU. No one (including Spanish marina staff) seems to know the reason for this malfunction.