Amazon enabled for his customers the possibility to receive alerts via email based on their billing situation.
Here is the official email:
Dear AWS customer,
We are pleased to announce that you can now receive billing alerts that help you monitor the charges on your AWS bill. Starting today, you can set up an alert to be notified automatically via e-mail when estimated charges reach a threshold that you choose.
To get started, visit your Account Activity page to enable monitoring for your AWS charges and set your first billing alert on your total AWS charges. Within minutes, you will also be able to set alerts for charges related to any of the individual AWS services or accounts that you use. Setting up a billing alert is simple. First, provide an e-mail address to subscribe to the alert, select the charges you want to monitor, and enter a threshold. Once you do that, you will receive a subscription confirmation e-mail from AWS sent to each address that you provided. Click the confirmation link to complete setup. Your alert will then be active and you will receive a notification as soon as your estimated charges exceed the threshold you chose.