Requires Steam. They also state "GameStop App must be installed in order to download software.", although if it's a Steam key you don't need to download it from them.
For some reason it allowed me to switch from showing AUD to USD, but won't let me switch back.
Okay, I can correct that - you don't need the app. You can install from Steam, using the key that is on your emailed receipt just below "Quantity" (and not incredibly obvious unless you enjoy reading receipts closely).
Came up as $19.83 USD and $20.38 AUD for me, although still not bad!
oh wait, either new link or it changed back. seems to show appropriate pricing now!
Same here. $20.38 AUD
Edit: Wooh! Came up as $12.80 USD when I tried again. Got myself a cheap copy. Thanks OP.
For some reason, after posting the deal I was able to order the game for $13.16, which was automatically converted to USD$12.80.
Weirdos - not that I'm going to complain too loudly.
And in further news, they don't just give you the key, you still have to download the app:(. On the bright side, I've apparently done this before with no long-term ill effects.