I think I would rather have things overload and go down than have a remote service be able to handle it but charge me a fortune.
I wish more startups would start up with this premise in mind, as at the moment I'm looking for specific services for my company, and I have the added task of trying to determine if the cool startup I'm looking at has a business model in place so that I don't have to worry about looking for a replacement for them next year when they close down.
(And yes, I'm looking for stuff which my company can pay for, not freebies)
Amazon does charge automatically and doesn't have a limit on the maximum amount I want to spend, so there is a risk there. But then, its not too expensive even if I get a lot of traffic.