You cannot change tracking numbers on ebay when you don't use Simple Delivery.   You can message your buyer with the tracking details.  If necessary you would be able to add the new number to an "Item not received" case if one was opened.   Payment may be held for 14 days.

Assuming the buyer paid the postage they will automatically get a refund for the unused label in 30 days.   I'm afraid there is no way of getting back the £15 you spent.