@patrick, I also contacted support and they replied that they are not billing for excess bandwidth. This is only for time being, they may charge in the future and will inform about this through a blog post as mentioned in this thread.
But to be on safe side, I am using this code ( http://pastebin.com/2vXMBaSi ) posted by another user to make sure I do not cross the bandwidth. I modified it a little, so that instead of giving the monthly cap in the script,I gave the daily cap ( that is monthly quota/30) , so my site will go down when daily limit is crossed for any particular day. Also, I changed the script, so that instead of shutting down the server, I would just rename the public_html folder and make the site unavailable. Next day when the daily bandiwidth is reset , the folder is renamed back to public_html, so my site will be auto up. I also made the script to write to a log, so I can know whenever this happens and I can decide on taking a larger plan.