What happens to my sales if the internet goes down?

You keep selling. Every sale is written on the device first and sent second, so even if the app closes a second later the sale is safe. The cashier sees a line in green saying it is saved on the device. When the connection comes back the sales go up on their own, in order, with no button to press. Your printer and your cash drawer sit on your own counter, so both keep working the whole time.