If you're using an Android device, you should be aware that some apps include in app purchases. You can disable this functionality to avoid random and unintentional chargers. Check out this FAQ to find out how to change your settings.
You can't disable in-app purchases on Android phones as is the case for iOS devices for example, but you can still lock these features with a PIN or password (which will be required for each of your purchases).
Note: On the latest version of the Google Play Store app, this option has been renamed as "Password - Use password to restrict purchases"
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