I know this is your business and livelihood, but realistically what can you, eBay, PayPal or auspost even do about it?

If adding registered post to the shipping cost isn't feasible, you'll just have to accept it as part of the business and minutely increase your product cost across the board to cover the occasional loss.

Like you said, buyers think it's only $6, who cares. People are scum and try to get away with what they can. And with these instances, they can. If you don't trust a buyer, block them from your sales.

I'm hoping eBay keeps a track of the number of INR"s people claim and flag ones who do it continuously. Maybe this is something you can ask them one day.

Other than that. Not much more you can do but vent online 😞