Shop Manager: Wholesale Sales & Stock App

The problem

My brother runs a seasonal watermelon wholesale business. Sales, credit and stock lived in a notebook: who owes what, how much stock is left, and what today's total was took manual adding up, and mistakes were easy to miss.

What it does

• Record a sale in seconds: customer, weight × rate per line, optional final price (the discount is worked out, never typed).
• Payments in cash or UPI, partial or full. A lump-sum payment is split across a customer's oldest unpaid orders automatically.
• Live stock from purchases, sales and wastage. A sale that would take stock below zero is refused.
• Purchases with landed cost (goods plus transport, labour and other costs).
• Daily and monthly reports, plus a today dashboard.
• Bills shared on WhatsApp when the customer has a phone number.
• Installs on the phone like an app (PWA) and updates itself with one tap.

How it's built

• Every write goes through a database function. The app never edits tables directly. Each PostgreSQL function checks the input, updates stock and money together in one transaction, and returns a clear error code.
• Security in the database, not just the app. Row Level Security and an allowlist mean only the two approved accounts can read or change anything, even with the public key.
• Nothing is ever deleted. Orders are cancelled, payments voided, and every edit is kept as a version, so the history can always be checked.
• Safe on a bad network. Each save carries a unique request ID, so a double tap or a retry can't create two orders.
• Two people, one stock count. Stock changes take a lock, so two sales at the same moment can't sell the same stock twice. Tested with two simultaneous sessions.
• Separate dev and production databases, a daily keep-alive job, and encrypted nightly backups through GitHub Actions.

Result

• Live since October 2026, used daily by the owner and me.
• 150 automated database checks covering money maths, stock rules, permissions and edge cases. All passing.
• Runs entirely on free plans: ₹0 a month.

Private business tool, demo on request. My work covered requirements with the owner, data model and business rules, development, testing and deployment.