I couldn't find much about that either. I found one small article pertaining to it but I think it would be best to contact Microsoft directly for an accurate answer.

1) Can I buy using my current balance instead of using credit card?
- No
2) Can I buy cards of Xbox Game Pass like Gold and Gift cards?
- Not yet, but it is planned
3) What is the deference between these 10$, 11$ and 12$ Xbox Game Pass digital cards? (They have same description)!!!???
- Nothing, just buy it from the Microsoft website