Not sure if it will work for QR code labels but have look in your postage labels section.
https://www.ebay.co.uk/ship/labels/my-ebay
At the top of the "more actions" drop down menu next to your item should be " print another label".
Click on it and a banner should come up that has a reprint link on it.
Click on the reprint link and it should bring up a page where you can open and download the label.