All you can do is use flat rate postage. Are you aware that if you list on the US site then you will come under the ebay seller protection policy rather than the Aus Paypal scheme so will have to not only prove that you have sent an item but that it has also been delivered. You will also have to pay FVFs on the total cost including postage. Your payments will go into your Paypal account in US$ and you will have to pay a conversion fee to change them to AU$ to withdraw to your bank account.