You can cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list). Opt out of Simple Delivery (see below) and relist with correct postage for the buyer to buy and pay again.
So there is an issue with opting out of SD for large items but see Rick's workaround solution on here:
https://community.ebay.co.uk/t5/Seller-Central/how-do-you-list-anything-bigger-than-61cm/m-p/7882486...
You can cancel the original postage label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Or go to orders, select Postage labels on lefthand side, then click on Actions dropdown menu for the label and select cancel or void Label.
If the buyer paid for a label it will automatically be refunded to them in 30 days.
@ssw-42