You can't opt out but if you've already sent the item by other means you can mark dispatched by opening the original Simple Delivery label (Get prepaid label > Get prepaid label and QR code). It will remain unused.
The option to change tracking number has been removed. You can message your buyer with the tracking details. If necessary you would be able to add the new number to an "Item not received" case if one was opened. Payment may be held for 14 days.
If the buyer paid for the original label the cost will be automatically refunded to them in 30 days. Or if you paid for it (ie. you had free postage on your listing) you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
You are selling new goods so should register as a business seller on ebay. Then you don't have to use Simple Delivery. You can buy your own postage.
