Start with the counter routine
List the actions staff need to complete during an outage: opening an order, adding items, recording a cash payment or printing a bill. An offline POS can store these actions on the device. Online payment confirmation, live stock across branches and message delivery depend on external connections, so decide how the interface should explain their status.
Make pending work visible
A bill saved locally and a bill confirmed in the central database are different states. Staff should be able to see which records are waiting to sync and whether anything needs attention. Each transaction needs a stable identity so a connection retry does not create another copy of the same sale.
Test the return of connectivity
Before launch, disconnect a working device, create orders and bills, then reconnect it. Check that records arrive once, totals reconcile and the pending queue clears. Repeat the exercise with two devices selling the same item. Agree how stock conflicts are handled and train staff on the steps to take if a sync fails.