add user registration and paypal payments to android app

i have an existing android app, which shall be extended with a) new user registeration and login b) once logged in a possibility to buy some things using paypal (i.e. integrate paypal sdk) c) show account status. json apis for registeration, login, account info retrieval and adding payment are available. mostly native controls will be used but some colors / backgrounds could be adapted to fit the rest of application color. Once started first working version shall be delivered in 3 days.