Copied from Mastercard Website;
Shop at over 100,000 U.S retailers and get fast, reliable, affordable international shipping with MyUS.
Enjoy great benefits when you sign up with Mastercard®, including a MyUs address to use at checkout with your favourite online stores, free Premium Membership for 2 years (US$120 value), free account setup (US$20 value), 30% off shipping for first month following registration, and 20% off shipping for remainder of 2 year membership.
Extra 20% off standard shipping rates
Free parcel bundling and repacking
Free 30 day parcel storage
Flexible shipping options
Also states Amazon US purchases can be made & shipped via this information
AU shipping calculator here
Mod: Myus has supplied the OP with a direct link for OzBargainers to signup to the 2 year premium membership directly, without a MasterCard required.