Using a Google Play Balance

If you have a  balance, and you go to buy something that costs more than that balance, there's no way to use all your balance and take the rest from another source. Instead you have pay the entire amount from the other source, leaving behind forever some small existing Play balance.

However

You can add to your Google Play balance, so there's enough, but it took me forever to find this option!  In the mobile app, Play menu, select My Account, then "Add payment method" (yes "Add payment method", ????? , does that even sound remotely like what I'm looking for? No, no Google, it does not), from the next screen, select "Buy Google Play credit".

And then you can get on with Game of Thrones.