SD labels cannot be cancelled. You have to cancel the transaction (via the dropdown arrow for item on your orders page - a refund will be issued) and relist with RM postage for the buyer to buy and pay again.
You can opt out of Evri via My eBay > Account tab > Selling > Postage Preferences > Simple Delivery > Courier Options > Edit. Or here's a direct link: https://www.ebay.co.uk/ship/prf
