It sounds like you didn't use the ebay Simple Delivery postage label so you've paid for the postage out of your own pocket.
To mark the item as dispatched open the original SD label. Then add the new tracking number via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That has to be done on a desktop browser.
Assuming the buyer paid for the original postage label the cost will be automatically refunded to them in 30 days.
