Microsoft Windows Store processes all purchases made via the in-game Shop. Available payment methods vary by country; listed below are the best available methods:
1) Credit cards, debit cards, or prepaid credit cards
2) Other online payment services, including (but not limited to) PayPal, Alipay, and INICIS
3) Microsoft gift cards

Note that,  not all payment methods are available in all locales or for all Microsoft services.
Payment Methods for the Windows 

IMPORTANT: Many of our players were in the same situation as yours, and were able to buy gems by purchasing the Microsoft Gift Cards! You may want to try it out!

For information on how to buy or use the Microsoft Gift Cards:

Gift cards from Microsoft

For more information, visit Microsoft Support