Automatic billing conveniently tops-up your credit balance when you have fewer than 12 credits remaining in your account. You can easily deactivate this feature by:
Accessing your account using desktop or mobile web;
Clicking on the three horizontal pink lines [☰] in the top-left corner;
Then on Settings;
Then Account;
Scrolling down and selecting Purchases; and
Disabling Auto Top-Up at the top of the page.