This is how to finalise that transaction.
You need to do this now, only on the Seller Hub
https://www.ebay.co.uk/sh/ovw
go to Orders >> Awaiting Postage / Print Postage label, and then click the box on the left of the item.
https://www.ebay.co.uk/sh/ord/?filter=status:AWAITING_SHIPMENT
From the Postage tab above, use the drop down menu there, if the option to mark as collected is not there, then select Mark as Paid and Mark as Dispatched.
Dispatched is eBay's wording for a collected item.
You should then be allowed to leave feedback, however, personally I would wait for the buyer to leave you some before leaving any. You do have 60 days in which to leave feedback.