You can do that easily. You just have to give more money to apple to buy the case and attachable battery pack.

This is not true. None of the Apple cases (or third party cases) give a flush finish to the entire phone. They just add a new, bigger, larger bump below the camera bump which lets the phone basically lie flat. It does not make it easier to smoothly fit into a pocket or anything like that, and the phone is still wobbly while placed face-up on a surface.