Cloud Functions or Basic request ?

Hi everyone, I'm developing a mobile app in flutter. Users should be able to add points to a Points collection (String userId, Int points) by redeem a coupon code.
How can I do this ? Do I need function or can I do that with client-side integration ?

Thank you for your time.
Vincent.

In your use-case you can do in at the client side

It would be better to do it with a function that way it will be easy to verify the steps and/or add more logic to it.

So, Every time a user enter a coupon you can send it to the function, the function will have the user id and the you can easily add it to the collection

Thank you very much.