If the parcel is going back to MISSGUIDED, Clarks, Jacamo, Simply Be, JD Williams or Disney Store you can print a label in store, free of charge, simply by entering their reference number (with no QR code required):
https://www.myhermes.co.uk/our-services/print-in-store.html
(That assumes that the store has a Hermes printer installed in it, of course).
Otherwise I would assume that the shop will have some way of reading your QR code. (Such technology is commonplace now. On most rail lines around here, rather than presenting a paper ticket, travellers simply show the conductor the QR code on their phone, which he then scans with the camera on his own phone. If QR codes can be accepted that easily on a train, it should be similarly possible to use them in a Hermes store).