You can use amp php library to get bucket size programmatically.
http://docs.amazonwebservices.com/AWSSDKforPHP/latest/#m=AmazonS3/get_bucket_filesize is an example of how to do this. Using the above example, you can create a simple function that gets the dimensions of all your buckets. If they exceed your size threshold, you can send yourself an email. If you are running UNIX, you can configure crontab to run this script every x times.
Amazon may offer bandwidth notifications via email. Did you conduct any investigation of yourself?
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