See if you can change the tracking number at View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser. If not generate the original prepaid Simple Delivery label and try again.
That will at least mark it as dispatched. If you still can't change the tracking the payment will be held for 14 days and then released.
The cost of the unused label will be automatically refunded to the buyer in 30 days.