When in store, use the built-in scanner to scan barcodes of the items you want as you drop them in your basket. You will see their prices and your running total in real-time so you can track your spending and stay on budget. No surprises.
Checkout lines are terrible, no one needs them. When you're done shopping, checkout and pay directly on your phone, by yourself, and entirely skip the checkout line. It couldn't be easier: with one convenient button you get your receipt and you're ready to go.
Head for the exit, show your screen to the friendly sales rep, and go!