I keep seeing a lot of people having issues with purchasing additional content through the Microsoft Store outside of NA.
Here is a quick breakdown:
Go to any e-retailer that sells Xbox Gift Cards NA (I use g2a)
Purchase said gift card using PayPal
Go to the Microsoft Store App
Click on the "..." on the top right
Enter your code
Purchase your item as normal
During this entire process, make sure your computer's region setting is set to US.
Edit: If prompted for an address, use this one that was provided in a previous topic on this forum:
702 West Johnson Street, Suite Madison Wisconsin53715-1007 United States