Not much. Major e-commerce sites get DDoSed all the time, and they have pretty sophisticated border network routers that can detect and deflect those attacks before they hit the actual servers. Or, like Google, the servers are more widely distributed than the attacking botnets.

It’s the smaller sites that are the most vulnerable, because they can’t afford to harden their networks that well.

None. E-commerce sites aren’t that important in the broader scheme of things, and even attacking thousands of them at a time would do little more than provide fodder for the news media.