You probably have already - but - check the address on the invoice through the eBay purchase.

Then check the 'ship to' address on the paypal details.

If an email directing you to ship to 'other' copy & keep.

Given it was Registered Post & admitted to being 'mislaid' by Aust. Post - PayPal cannot lay blame on the seller.

The buyer has the Registered item number & should follow up the where - while - you already have proof of postage with the same as far as PayPal is concerned. 

Think PayPal should wake up to the difference between proof of postage & proof of delivery - they are not the same and they cannot have it both ways.