add square payment gateway to a mobile app.

I have a mobile app but unfortunately there is no payment gateway. I would prefer to have Square payments because I already have an account with them and I get Instant deposits through them. The app was make using Flutter. I just need someone to add Square to it so I can accept payments through the app. If you are able to remove the "cash on delivery" option that would be awesome too. If you need any other information that I didn't list on here feel free to reach out to me.