I use budgets in AWS to send me a text when I get to a certain limit. You could potentially use an SNS topic to take action and shutdown resources I suppose. I guess if you are always traveling or can't get to a computer this could be an issue for you. I can't imagine there are that many people seriously using AWS who could suddenly run up a huge bill that can't respond to a txt or have an friend, coworker, employee or automated task do so.