This is what I do:
Buy Xbox gift card (in USD) from Amazon.com
Go to Xbox.com, login, change region on website to US
Redeem gift card
Go to game on Xbox.com, you should see the price in $, purchase using the credit in your wallet
Save money
Worth noting that you may need to change the region on your console first and set up a fake billing address, I did this ages ago so I'm not sure if it's a required step as the above works for me without ever changing the console region now, but possibly because I set up the US profile a while back.