If you bought your own postage instead of using Simple Delivery first mark the item as dispatched by opening the original prepaid SD 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.
If the buyer paid for original SD label the cost will be automatically refunded to them in 30 days.