I know others on here have expressed no sympathy for the buyer,  but you offered express post knowing that Australia Post don't guarantee delivery at the moment.

And your listing would have listed an expected delivery date for that item.

So yes technically not your fault as it was in Australia Posts hands,  but don't offer a service which isnt working,  you were happy to accept the higher fee from the seller.

Do the honourable thing and refund the buyer.