Is it on this ID? Because nothing is coming up as a recent sale. Anyway, sending revised invoices is glitchy at the moment so it's unlikely you'll be able to do that. But if you are you can invoice the buyer for your own postage cost and buy postage from your post office. Just remember to add the tracking number and mark dispatched manually.
If you have problems with that cancel the transaction altogether via the dropdown arrow for the item on your orders list (a refund will be issued). And either relist with direct postage to buyer's country for them to buy and pay again. Or just relist for someone else to buy. If the latter you need to put the first buyer's ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock
