After this amount of time the only reason a case can be opened is for a credit card chargeback. The email from Paypal will say that the buyer has instigated it. The buyer would have gone to the bank and then the bank contacts Paypal.

For seller protection to apply for a credit card chargeback you need to provide documentation from Australia Post showing the buyers delivery address - an ebay invoice does not provide sufficient proof for a credit card chargeback case.

You could try contacting the buyer and asking why they have done it. Sometimes in these situation the buyer has forgotten what they purchased and will offer to repay the money. You will most likely also be charged a $15.00 fee by Paypal so you could ask the buyer to cover that as well.