That's true about EC2. However if you're using an E-commerce framework that is done well, for example a Java one that uses JDA/JDO for storage and abstracts all

That's true about EC2. However if you're using an E-commerce framework that is done well, for example a Java one that uses JDA/JDO for storage and abstracts all I/O into a datastore, you could very well use it on App Engine. And also, App Engine gives you scale confidence and zero maintainability.