I need a straightforward Android-only mobile application whose first release focuses on a rock-solid payment integration. The core flow is simple: users open the app, choose the product or service, and pay directly inside the app without being kicked out to a browser. I am open to the payment gateway you feel most comfortable with—as long as it works smoothly for Indonesian users and passes Google Play requirements. Please keep the codebase clean and documented so I can extend the app later with other features. Deliverables I expect from you: • Production-ready APK and Play Store-compliant bundle • Full source code (Java, Kotlin, or Flutter—your choice, but stick to one) • Integrated, fully tested payment flow running in both sandbox and live modes • Clear hand-over documentation: build steps, environment variables, and how to switch keys If you’ve built similar Android payment apps before, share a link or short demo so I can see your approach. Let me know your estimated timeline and any gateway fees or technical constraints I should be aware of, and we can get started right away.