Are you using the app or mobile browser? Then, switch to a PC / laptop and use Chrome and proceed this way:
If you are on My eBay Selling, go right of the item sold and from More Actions select Refund this item. Be careful to then add the correct amount of refund you wish to make, or you will refund the whole amount.
If you are on the Seller Hub, click on the item in Orders >> Posted and Awaiting your Feedback. Then top left click on View Order details. Far right from More Actions Drop down menu, you will see Refund this item.
One tip, before you send the partial refund, mark the item as dispatched, there is a quirk in eBay's programming if you send a partial refund first, you then cannot mark as dispatched.
