handle cart, order scheduling, payment gateway and bug remove in our flutter app -- 2

Hello Dear, we have ongoing flutter application (Online shopping) project and we want to improve stability of our application for that we need to develop below mentioned features.
1. Cart handle (backend api ready).
2. scheduling of order handle.
3. easypay payment gateway integration (Optional).
4. design issues fix
5. minor issues and bug fixing.

Please apply with timeframe to complete point no 1 to 3.