It seems it is not possible for customers to download a label from the returns portal, on print. In the case that a customer does not have a printer, the only solution is for the customer to contact the retailer. The retailer can then download it and send it to the customer.
This seems a bit admin intensive.
It would depend on carrier, but a download option for the customer would be advantageous. Equally, if it features a QR code for the post office / drop off point to scan and print the label for the customer.