Seems like a pretty solid deal for a digital copy. It might be hitting Game Pass soon, regardless this is a great deal.
Credit to OverburdenedWarrior for directions on how to claim this deal.
I've managed to grab it, but as others have noticed, you can't use your Australian cards/PayPal. Instead, you'll have to purchase a gift card and redeem it on the console itself, after switching your location in the settings. It used to be a lot easier to purchase from the overseas stores, but over the last few years they've been cracking down on it.
Here's a list of stores that sell credit: https://www.xbox-now.com/en/gift-card-shops
Once you've brought it, go onto the console, into settings and find location. Change it to the US and follow the prompt to reset the console. Once you've done that, you'll be able to redeem the code and purchase the game. Once brought you can safely change back to Australia.
If you haven't brought from the store before you'll probably need to add an address, even to use a gift card. Try to find a hotel in a state without sales tax.
Also see this thread for purchasing Xbox games from a non-Australian Xbox Store page: