Google App Engine provides generous free quotas for your app and additional paid quotas.
I always enable billing for my GAE apps even though I rarely exhaust the free quotas. Enabling billing and setting paid quotas does not mean you have to pay anything and in fact increases what you get for free.

Here is a screenshot of the billing panel:

GAE lets you allocate a daily budget to the various resources, with the minimum permitted budget being USD $1. When you exhaust a free quota you will only be charged for the budget allocated to it. In the above screenshot I have allocated all my budget to emailing, but since my app does use the Mail API I can be confident this free quota will never be exhausted and I will never pay a cent. For another app that does use Mail I have allocated all the budget to Bandwidth Out instead.

Now with billing enabled my app:

• can access the Blobstore API to store larger amounts of data
• enjoys much higher free limits for the Mail, Task Queue, and UrlFetch API’s - for example by default an app can make 7000 Mail API calls but with billing enabled this limit jumps to 1,700,000 calls
• has a higher per minute CPU limit, which I find particularly useful when uploading a mass of records to the Datastore

So in summary you can enable billing to extend your free quotas without risk of paying.

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