To mark the item as dispatched open the original 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.
It seems scary to send an expensive item using £3.42 postage but Simple Deilvery automatically covers sellers up to £750 for loss or damage in transit.
eBay will class you as a new seller and may hold the payment for up to 30 days regardless of the above process or feedback.