Need advice for a refund from a third party seller on Amazon

Hello, so I ordered a belt from what apparently was a third party seller on Amazon Germany and when I received it it wasn’t the quality that I expected so I decided to send it back for a refund.

This is when I realized that something was different since I had to print out the return shipping label myself and also pay for the shipping myself as well.

The problem is however that the seller refuses to issue me a refund since they say they haven’t received anything and DHL say they don’t provide tracking numbers for small packages. I still have the receipt from when I paid for the shipping but nothing else really as evidence since I didn’t expect such a scenario so I would appreciate any help with this, cheers.