Best structure for my "Online Shop"

I think you can use hmvc pattern. It can help you to manage everything, in my project I created two controllers, BaseController and InterfaceController. I put them in app\Controllers then I set some default config.

I extends InterfaceController for handle interface request, and BaseController for do some action. Its also good for you if you can one class one function.

Simple doesn't mean for today, simple can be complex in another day