Hey dlemmon;
Some information for you. If you decide to purchase the iPhone X in Australia, make sure you get a GST refund form from the store you bought the iPhone from. Make sure you save the store receipt and the box the iPhone came in. You can save more than 10 percent.
Ref: https://www.border.gov.au/Trav/Ente/Tour/Are-you-a-traveller
...yes, but technically you are required to declare and pay customs for the full price of the phone upon arrrivsl in the US