Once a Pass is added to Passbook, your customer can retrieve it by opening the Passbook app. Inside Passbook, Passes are stacked on top of each other, leaving only the header visible. This is why we encourage you to add a logo to your Passes, so that your customers can find the correct one in a instant.
Then, just touch a Pass to make it appear completely.
If your provided relevant locations (geofencing) or a relevant date in the Pass Designer or using the API, a message will appear on the iPhone lock screen when or where you said it should.
People just have to swipe the message from left to right to make the Pass appear.
How you use the Passes at your shop is up to you: you could scan the barcode, read any relevant data inside the Pass...