To mark the item as dispatched open the original prepaid Simple Delivery label.
Then change the tracking number via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That has to be done on a desktop browser. Even without doing that the payment will be released but ebay will put it on hold for 14 days.
Afterwards if you paid for the original label you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Or if the buyer paid for it it will be automatically refunded to them in 30 days.