Everyone has a story of a counter frozen while a customer waits with cash in hand. A POS that needs the internet for every sale is a POS that fails at the worst moment.
Price it the same way
The risk with offline selling is that the slip and the final invoice disagree. Symtex's desktop app uses the server's own tax engine, imported rather than copied, so the offline figures follow the same rules.
A slip now, an invoice later
The customer gets an OFFLINE slip with a provisional number. When the connection returns, the sale uploads and the server re-prices it. The slip number is saved on the invoice, so you can match the two.
What can still go wrong, and how it is shown
Another counter may have sold the same unit, or a discount may exceed the user's limit. Those sales are never dropped: they stay in the queue flagged Needs attention, with the reason, until someone decides.
Why offline sales are paid in full
Once goods have left, nothing can be refused because of a credit limit. So offline sales are cash, bank or wallet, in full.