Package returned to Amazon warehouse in Madrid, but I'm not a FBA seller. Can't get package back.

Hello, I sold a item to someone in Spain. This client returned the package to a Amazon warehouse in Madrid. But I am not a FBA seller and I don't have a return address in Spain. Now I can't get the item back. I opened a case and started a chat with Amazon but they keep telling me I should ask the courier. They are called Celeritas. I can only find a contact form online. But they don't respond. How do I get my package back? And how is it possible the client returned it an Amazon warehouse?