The system doesn't let me upgrade my iCloud storage

My iCloud storage is full so I have been trying to upgrade it but as I enter my card details, I get the error message below: "The credit card you entered is not valid in Germany..." Only issue is: my card is definitely valid in Germany since it is from a German bank and I use it daily. My iTunes and iStore accounts are up to date with my German address so I don't know what's wrong. This is really frustrating, any help (or even better, contact to an actual customer service!) is appreciated. Thanks