Advanced Android Billing App

I’m building a single Android application that handles every step of small-business billing in one place. The core of the project is an all-in-one module that lets me:

• generate professional invoices on the phone,
• process customer payments in-app, and
• track expenses in real time.

Beyond those essentials, I also need the app to exchange data smoothly with my existing accounting software and CRM so client records, invoices, and payment statuses stay perfectly in sync. If a standard connector is not available, a lightweight REST integration is acceptable as long as it preserves data accuracy and runs securely.

The finished APK (plus full source in Kotlin or Java) should:

• run cleanly on current Android versions down to API 24,
• store data locally with automatic cloud backup,
• offer a modern, intuitive UI that matches Material guidelines, and
• expose clear documentation for every endpoint and integration point.

I’ll sign off once the APK installs without crashes, all three core features work end-to-end, and sample data flows correctly to my accounting and CRM tools.