A buyer has opened a dispute claiming they haven't received an item I sent back in September.  The buyer is from Russia.  I stated on my listing that shipping is via standard airmail which has no tracking and that if that if they wanted tracking it would be extra cost.  The buyer did not opt for tracking so it was sent standard airmail.  I have the Australia Post receipt of when I sent it but when I go to the resolution centre, there is nowhere to attach the photo.  It only lets you type a text message.

I've never had a buyer open a dispute before so I'm not sure what to do.  What would happen in this case?  Is ebay likely to make me refund the buyer as there is no tracking which I think is unfair as buyer chose not to pay for tracking?