If there is a problem with the label (specifically with the QR code, which may well be down to a fault with the printer rather than the code itself), that means that it will not scan, that also means that it will not scan at the point of delivery, so delivery confirmation will be unable to be confirmed.
You will be relying on your seller being honest and acknowledging that they have recieved it. If they decide to be difficult and deny recieving it (or if it really has gone missing in the system), a Proof of Postage will not be sufficient to show the ebay system that you had sent the item back. You would then need to contact ebay customer services and try to persuade them that you did return it, but I would not like to pretend that it would be anything other than an uphill task to do so 😞
