If you are using Google App Engine for deployment of your production setup, then based on your application requirements you may need to enable billing in Google App Engine.
An application without billing enabled is allocated 500 MB of persistent storage and enough CPU and bandwidth for about 5 million page views a month. The Admin Console's Quotas page lists every quota and provides a breakdown of how much your application is consuming per resource.
Your App Engine Application is basically free to serve applications with daily quota limits, you can check your Usage in your application's dashboard in Google App Engine. The Quota limits reset every 24 hours. But if you use up all the resources within 24 hours then your application will stop serving. To avoid this you will have to enable billing.
To Enable Billing in Google App Engine:
Click on Settings Next to Billing Status under the graph (or)
Click on Billing Settings in the left side menu under Billing.
Click on Enable Billing Button, now you will be presented with the various options / settings for billing. Choose according to your requirements, and pay with google check out.
Please go thru the following links to know more about billing and quotas