I would like to know the details of why we must end a dua by saying “Ameen” and why we must not end it with “in shaa Allah” so that I can explain this matter to a sister who used to end her supplications with “in shaa Allah” instead that with “Ameen.” May Allah reward you with goodness.
Saying Ameen after Duaa means asking Allah to answer your supplication.
We do not say In Shaa Allah (if Allah wills) because this implicates that we are not desperate for Allah to respond, it does not give the meaning of affirming your needs.
The Prophet, peace be upon him, guided us to affirm the way we ask Allah by showing our dire need to His response, and never to say Allah forgive me if you will.
For example: We need to say Allah forgive me or I will perish, i cannot live without Your forgiveness and so on…