Operation: The hard wallet stays in a private vault in a secure location and a web based Point of Sale Kiosk presents the QR code total to the customer.
You've just described what every single real seller does, do you imagine they are carrying around PoS with the hardware wallets on it and the private keys there? They just have a listening function that detects the payment from x has been made to the y address displayed and the value matched.
Besides most sellers are interested in swapping directly the coins for $ t not to get screwed by the fluctations in the markets, so they don't touch the coins at all.