If they've paid anything you won't be able t send an invoice. If they haven't paid see if you can send another amended invoice via the dropdown menu for the item. If not you could cancel the current transaction (also via the menu) and relist with postage for them to buy again.
If you prefer not to post it you can just cancel the transaction and leave it at that. If so use the reason "something wrong with buyer's address" - ie. they're too far away to collect. Also put their ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock