Point-of-sale integration is unforgiving because the failure is public. A slow response or an error is not a logged exception; it is a queue of customers watching a cashier apologise. That constraint should shape every design decision.
Assume the network will fail. A till that cannot complete a sale without a round trip to your servers will stop selling during every connectivity blip, and connectivity blips are routine. Offline capability with local queueing and later synchronisation is not a nice-to-have in this domain — it is the requirement.
That makes conflict resolution unavoidable. Two tills selling the last unit of stock while offline will both succeed locally, and the reconciliation has to have a defined answer. Deciding that answer up front, with the business rather than in code, avoids a class of disputes at month end.
Invoice format is a compliance matter, not a design one. A retail invoice in India carries specific required fields, and where e-invoicing applies, the document must be registered and carry the resulting reference and QR code. Getting this wrong surfaces during an audit rather than during testing.
Peripherals are where estimates fail. Thermal receipt printers, barcode scanners, weighing scales, card terminals and cash drawers each connect differently and each vendor implements the standards loosely. Budget integration time per device model, not per device category, and test with the exact hardware the stores actually have.
Finally, design the cashier's screen for speed over completeness. Every additional field, confirmation or scroll is repeated hundreds of times a day by someone with a queue in front of them. Software that is elegant in a demo and slow at the counter gets worked around, and the workarounds become your data quality problem.
