Hi,
I'm Rodrigo and I'll help you.
You can use the Xbox Gift card to pay on the Microsoft Store.
About the taxes, if add a 30 € Gift card to buy a 29,99 € game you may need to complement the value to pay the taxes, you can pay the difference using PayPal, a Credit card, or by adding another gift card.
You can add the gift card directly on the Microsoft Store payment page