Flutter Recharge & Bills API Integration

I have a production-ready Flutter app and I’m ready to expand its feature set. The next milestone is to add a unified recharge & bill-payment module so users can:

• Top up mobile numbers
• Pay electricity bills
• Pay water bills

What I need from you
1. Plug a reliable recharge/billing REST API into my existing codebase (I’m open to Paytm, Mobikwik, Freecharge, or any other provider you recommend).
2. Write clean Dart code that covers authentication, secure token storage, transaction status polling, and graceful error handling.
3. Expose a simple service class and callback structure so my current UI can trigger payments and receive success/failure responses.
4. Deliver concise setup notes (API keys, test environment, production switch-over) so I can replicate the build locally.
5. Provide basic test cases or Postman collection proving all three services work end-to-end.

The app is built entirely in Flutter with state management already in place, so you’ll focus only on the network layer and minimal widget wiring. If you’ve done payment integrations before and can turn this around quickly, let’s get started.