eBay don't usually use Yodel but if it's incorrect you can cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
Or if you want to use cheaper postage than anything ebay are going to offer you could buy and pay for it yourself. Bear in mind it's the buyer who will pay for the ebay postage.
Anyway if you decide to buy it yourself to add the new tracking number you still need to generate the original prepaid label. That will also mark it as dispatched. Then change the tracking via the More Actions dropdown menu or View Order Details > Postage > Track Package > dropdown arrow. That needs to be done on a desktop browser.
Or your postage may not have tracking in which case payment will be held for 14 days. If buyer paid for the unused ebay label it will be automatically refunded to them in 30 days.
