Your final price for stuff you buy at Microsoft Store is the same regardless of how you pay for it, whether you're using a gift card, credit card, debit card, etc. That includes taxes.
The exception would be if you live somewhere (e.g., Denmark and Sweden) where the tax is collected when a gift card is bought, then you won’t be charged tax when you buy stuff with it.
There are no fees for having a Microsoft account, putting money into it, or using it to buy stuff from us. In other words, it’s free!