You might have to cancel and relist opting out of Simple Delivery so you can print/write your own label and buy your own postage.
If you decide to do that you can issue a cancellation via the dropdown menu for the item on your orders page. Use the reason "Something wrong with buyer's address" or "Buyer asked to cancel". If they've paid the refund will be issued in the process.
Then relist. You need to opt out of ebay's new Simple Delivery during the listing process. Click on the postage options icon on the right of the postage section and remove details of parcel weight and dimensions. Then you should be able to add what postage you want.
If you buy postage outside of ebay remember to mark dispatched and add the tracking number manually via the dropdown menu arrow for the item on your orders page.
If that all seems too much of a faff you could cancel and put buyer's ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock
And relist for someone else less complicated to buy.
