There isn't a solution now that the item has been bought, paid for and delivered.
It was originally listed for postage, therefore your buyer cannot receive any collection code to give you, and this is the only way for you to mark the item as collected / delivered, and to facilitate a faster payout.
You will need to wait a minimum of 14 days for this payout, as there is no way for you now to mark as delivered / dispatched, or for the buyer to mark as delivered, except to leave feedback and this can only release payout for items of the value £25 or less.
@alastairthomson