Customer protection organisation

HI.

I'm a seller (Not FBA) and I sent the item to my customer. He gave me the address for Amazon Locker.

I can see that DHL delivered an item to the address. Unfortunately the customer said that he has no access to the locker. He opened an AZ case. and amazon sent him money back.

I contacted amazon a few times but they of course they are trying to get rid of me.

Since I can't get anything done with Amazon I decided to contact an external consumer protection organization or a small claims court. Hence my question. I have Amazon registered in the UK but my client is from Italy. Where should I contact the organization in Italy or the UK?