The best thing to do is cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
If you need to opt out of Simple Delivery to use the postage you need see the solution on here:
https://community.ebay.co.uk/t5/Seller-Central/how-do-you-list-anything-bigger-than-61cm/m-p/7882486...