Once you've added the products to the shopping cart, tap the green bar at the bottom of the screen.
Tap the green bar again to proceed to the payment screen.
At the payment screen, tap on "Save Order," right below amount received. Press that button to complete to pause the sale.
That's it! You have now paused the sale; you can resume from where you left off at any time. On this screen, you can also open the list of all your paused sales.
You can find all open orders on the main menu.