Here's the deal with US post office boxes and express couriers, such as FedEx, DHL, and UPS.

The express couriers do not deliver to PO boxes, so anything you order would have to be delivered by US postal mail. To top that off, anything forwarded outside of the US by the US Postal Service would be charged at the regular rate anyway. That means you would have to pay shipping twice, once from the retailer to your PO box, then from your PO box to Australia.