Let the chat app be a chat app. Let the payment app be a payment app.
Or, to put it another way, do you recognize that a chat app and a payment app have different incentives? Combining them means now your chat app has payment app related incentives. Do you see this as a good thing? If so, why?
When was the last time you saw an mobile app taking advantage of another one apart from tre camera? The only and most common time Android apps communicate is with Google Play Services.
Context switching on mobile or GUIs in general also don't follow this principle. Modern GUIs integrate plenty of things (file explorer, image editor, basic camera, etc) becuase mobile package management doesn't resolve dependancies automatically or allow for a great deal of UI customisation. Especially in a messaging client imagine not being able to send photos, videos, audio, or files!c
GUIs are here to stay so until we figure out a composable and we'll adopted GUI solution "doing one thing well" is not really an option.
I do this all the time when I use mobile payment apps. Website has a link that opens the app, I do my payment, back to the safari afterwards. It also works from within other apps.