Your fees will be changed into Au$ and added to your bill in the usual way. You will have to use flat rate postage as you will not be able to use calculated as that only applies to the US postal service, you wiol also have to have proof of delivery, not just proof of posting as you do with Australian buyer protection. Your payments will all be in US$ so you will have to set your Paypal account up to accept that currency and will pay a conversion charge when you go to convert it to AU$. As you get 40 free listings on the au site it may be better to consider using more than one id if you want to sell more than 40 items a week.