If you have a shop as well as an online store, the worst outcome is two stock counts and two sales figures. Trinavo's point of sale sells from the same inventory and the same product list.
These tutorials cover setting the register up and opening it for the first time, taking a sale at the counter from picking the product through to payment, putting a customer on a sale, selling with a barcode scanner, and giving a member of staff a cashier role without opening the rest of the admin panel to them.