bugs remove and updating features in our flutter app

We have ongoing live project on fruits and vegetables in flutter as android app. backend designed in java swing with MySQL database.

Developer should know frontend as well as backend
Developer need to design & develop frontend in flutter
Developer need to develop backend api as well as required or api may be provided on request.
Development should be bug free and optimised.

Features need to be develop.
1. Time slot management need to be implemented in application.(when user place an order there should be times slots to be choose.)
2. dynamic delivery cost should be there. (depends on time slot and distance).
3. Native payment integration with Eazypay pay (Right now razorpay flutter working).
4.order tracking option.

Developer need to work on remote system.