HOW TO BUILD THE SALES MODULE STEP BY STEP.

HOW TO BUILD THE SALES MODULE STEP BY STEP.

The sales module is the most used part of your POS system.

A cashier will interact with it hundreds of times every day. It must be fast. It must be intuitive. It must handle every scenario without breaking or confusing the person behind the till.

Step 1 — Design the sales screen layout first

Before writing any code, design the screen the cashier will use.

A good POS sales screen has three zones:

Zone 1 — Product selection

✅ Search bar — always focused and ready for barcode scanner input

✅ Category buttons — quick access to product departments

✅ Product grid or list — browse by category

✅ Recent products — last 10 items sold for quick reselection

Zone 2 — Current sale

✅ List of items added with quantity, unit price, and line total

✅ Subtotal, discount, tax, and grand total clearly displayed

✅ Ability to change quantity or remove item easily

Zone 3 — Action area

✅ Payment method buttons — Cash, M-Pesa, Card, Split

✅ Discount button

✅ Hold sale button

✅ Cancel sale button

✅ Customer select button

Design this on paper before you build it. Show it to the client. Show it to a cashier if possible. Get feedback before development begins.

Step 2 — Build product search and addition

The search must handle barcode scanner input, keyboard text search, and category browsing simultaneously. When a product is found, add it to the sale array, increment quantity if already present, calculate line total, and update sale totals.

Step 3 — Build discount and payment processing

Discounts check against user role permission before applying. Payments must handle every method and every combination. Split payments record each portion separately and only mark the sale complete when all portions are confirmed.

Step 4 — Save the transaction in a database transaction

Use a database transaction. Create the sale record, sale items, payment records, and stock movement records together. If any part fails, everything rolls back. Never save a partial sale record.

Step 5 — Generate the receipt

After successful save, generate receipt data, send to printer, send SMS if customer phone available, display confirmation, and clear the screen for the next transaction.

Step 6 — Test every edge case

✅ Product with zero stock

✅ Customer credit limit exceeded

✅ M-Pesa prompt sent but customer does not respond

✅ Printer offline when receipt should print

✅ Internet drops during M-Pesa callback

✅ Duplicate barcode scan

The speed test

A standard cash sale — one item, cash payment, receipt printed — should complete in under 20 seconds from product scan to receipt in hand.

An M-Pesa sale should complete in under 45 seconds including STK push and customer PIN entry.

If your sales module is slower than this in testing, it will be frustrating in a real busy environment.

Is your sales module fast enough, complete enough, and tested enough to go live?

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