As a seller, who has sold numerous items to buyers in Australia, I doubt the seller was trying to rip you off. I have sold things that the postage to Australia was all over the place. And the items were all about the same weight. One time I sent something, it was about $27.00 for postage, the next time it was $11.50, etc. I don't have a clue why, as I sent the items Air Parcel Post, and not Global Priority. My only explanation is there is something funky with the rates at the post office. And again, the items I sold to Australian buyers were all about the same weight, ie. shirts, dresses etc. I'm sure the seller was just trying to cover her costs in the rate she charged you and wasn't trying to scam you on purpose.
But then again, this has just been my own person experience with postage to Australia.