Oh the joys of international selling, especially clothing.   Am surprised you had the item authenticated by VP of production prior to selling, how much did verification cost or was it a freebie.

Anyway you have authentication for a dress, but no way of verifying that is the dress received by your buyer, while that may seem harsh, that is how ebay will see it, so pointless trying to fight the claim.

There are services available, but quotes all over $100 for 1kg.

You could try  USPS,  while I can obtain quotes and  flat rate costs from  US Post, I have never actually had to purchase from Australia, you would need to make sure the service included tracking and customs declaration, from memory starts at about $45

Otherwise cut your losses, or maybe offer a partial refund.