Let's start by going an app you want to buy. You'll click on the price button and the next window will be the details of the information the app will access if you accept. Once you've accepted the information the the app has access to you will be greeted with the payment option windows (popup).
If you were to click on the title right bellow the name of your app you be given 2 options. Payment options and Redeem. Selecting Payment options will bring you to all the various methods you can use to pay for an app.
- Carrier Billing
- Credit Card
- Google Play balance
Here's the screen shots in order of what you should see if you follow these steps: