Android Khatabook-Style Reminder App

I run a small retail business and need an Android application that works much like Khatabook but focuses on one thing: nudging customers to clear their pending balances. Each customer’s outstanding amount should be visible at a glance, and the app must let me fire off polite reminders by SMS, WhatsApp, or push notification on a schedule I choose.

Core flow
• I create a customer entry, add or edit their balance, and the app instantly updates a running ledger.
• When a balance crosses a date I set, the system sends an automatic reminder and logs what went out.
• If the customer pays, I record the payment (manually or through the integrated gateway) and the balance adjusts.

Essential features
• Android-only build, coded in Kotlin or Flutter.
• Secure payment-gateway integration so customers can settle dues straight from the reminder link.
• Full customer transaction history reachable from each profile and exportable to CSV or PDF.
• Push notifications for both reminders and real-time payment confirmations.

Acceptance criteria
1. Smooth installation on Android 10+.
2. No crashes during add, edit, or payment flows after 100 consecutive transactions in testing.
3. Gateway sandbox test shows successful, failed, and cancelled payments reflected instantly in the ledger.
4. Reminder scheduler supports daily, weekly, or custom date picks and logs every message with a timestamp.
5. Code, build files, and concise setup notes delivered via Git.

If you have already built something similar, a quick demo APK will help us kick off faster.