tl;dr I'm super confused by a series of events around an item I sold a month ago that has been returned to me with absolutely no messages from the seller and I don't know what is going on. 

So the item was purchased on the 1st of September and delivered to the buyer on the 6th. On the 21st of September I received an email from Paypal saying that they had recently informed me that the payment was made without the account holder's permission / was the result of a transaction error (I had received no notification from PayPal prior to this email and assumed it was a phishing attempt).

On the 24th of September I received another email stating that the case had been decided in my favour. I was pretty confused by this, as I hadn't done anything and I still hadn't received any messages from the buyer. Today I received a package in the mail which contained the sold item in it (just in a paper envelope - when I had sent it in several layers of bubble wrap and some cardboard). From the tracking number It seems it was sent back to me on the 24th of September shortly before the email from PayPal telling me the case had been decided in my favour was sent.

I have no idea what is going on or how I should proceed, is the buyer expecting a refund or something (despite never contacting me through ebay)?