Billing, Payments, POS and Retail Arithmetic
This domain tests the checkout: what a valid bill must contain, how payments are accepted and reconciled, and how quickly and correctly you can do retail arithmetic under time pressure.
Billing and POS
- A GST tax invoice must show supplier name and address, GSTIN, unique invoice number and date, customer name where registered, HSN/SAC code, description, quantity, taxable value, GST rate, CGST/SGST (intra-state) or IGST (inter-state), total and signature.
- POS components: billing software, barcode scanner, cash drawer, card and UPI terminal, receipt printer, weighing scale where needed.
- MDR is the merchant discount rate charged on card acceptance; UPI person-to-merchant is zero MDR by policy, which is why small stores prefer QR payments.
- End-of-day: count cash against the system total minus the opening float, match card and UPI settlements, record voids, refunds and petty expenses.
Arithmetic worked example
MRP is Rs 1,120 including 12% GST, and the store gives 10% off MRP. Selling price = 1,120 x 0.90 = Rs 1,008. Taxable value = 1,008 / 1.12 = Rs 900; GST = 1,008 - 900 = Rs 108. Customer pays by Rs 1,500 cash, so change = 1,500 - 1,008 = Rs 492. Always discount first, then extract tax from the discounted price.
Traps
- Adding GST on top of an MRP that already includes it.
- Rounding each line item instead of rounding the bill total.
- Ignoring the opening float, so the cash drawer never reconciles.
- Approving a refund without a bill check or manager authorisation, which opens a fraud route.