Customer Payment Flow

It was my old work. I started the problem that in offline shopping such as retail shop. sometimes customer have to wait in the long queue for there cart checkout so, I decided to design a flow where the user can add his items into the store app. where the user can scan the product and do online payment of that product and download the receipt and show to the door at the time of exit.
the benefit of this flow, user don't need to stand in the billing queue and feel offline shopping experience like online.