Is the item being sent via GSP? The seller is correct; she only sends domestically and the parcel is then repackaged and prepared for international despatch at the Pitney Bowes warehouse.

If the seller has already despatched the item, it’s too late for her to cancel. PB won’t be contactable for you to change the address. All you can do is see if you can redirect the parcel once it’s en route to Australia.

NOTE: the GSP is relatively costly but very very slow shipping.

You can't cancel the order only the seller can

The seller is 100% correct, they cannot change the address after the buyer has twice confirmed it as correct (as you do when buying) as per eBay policy (and this is no matter what the reason when the buyer gives the incorrect address, that is all the is relevant in eBay's eyes

Sellers must post to the address the buyer provides at the time of purchase 

Where is the seller? (are they in the US?) Of course if she is, the price for her will be US dollars

You will just have to be patient for the seller to respond

And in the meantime, make sure you are giving the right address before confirming purchases 

Only the seller can cancel the order.

You were afforded the opportunity to change your address at checkout.

You will need to message the seller again & hope they reply. The seller is able to cancel as problem with buyer's address.