[eluser]xwero[/eluser]
It makes no sense. It doesn't matter if you put it in the controller or in the post_system hook the user will not notice. Technically using a hook is slower than putting the code in the controller as the hook file needs to be loaded too.
Hooks should be used for actions that happen on (nearly) all pages as they get executed on every request.
if you want to separate the amazon functionality, you could create a library you call whenever it's needed.