You can just mark the item as dispatched this wording was used for a collection item. There is no other way as a collected item now needs to be finalised as collected, only with that 6 digit code, which your buyer would not have received, this not being listed originally as a collected item.
You need to finalise the transaction now only on the Seller Hub here:
https://www.ebay.co.uk/sh/ovw
Go to Orders >> Awaiting Postage, click on the item then top left on View Order Details.
Far right from More Action drop down menu click on ' Mark as Dispatched' .
@bluecarracer