To be honest it would be best to void and receive a refund for that unused label, and take the item back to the Post Office for them to weigh it, and you pay for over the counter postage, that way you'll know you have paid for the correct postage.
Enter tracking number when you mark the item as dispatched.
To void your label, you may first have to wait, 24 hours for it to appear.
Find your label with this link . https://www.ebay.co.uk/ship/lmng/labels-history/seller-hub and to the left of the sales record there is a drop down menu under 'Actions' click on that , then click on 'Void Label'. The refund will take between 5 - 7 days to process.