If you did not make the partial refund through eBay, because there is a quirk in the system, then you should be able to mark as ' dispatched' this way. It needs to be done 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' . That used to be, and can be now eBay's wording for a collected item.
@longmuir2006