When you buy your own label you still need to generate the original prepaid label to change the tracking number and mark as dispatched.
After you do that go to View Order Details > Postage > Track Package > dropdown arrow and change the tracking. That has to be done on a desktop browser.
If the buyer paid for the oriignal label it will be refunded to them automatically in 30 days.
Even if you don't manage to do the above the payment will be released anyway in 14 days.