Confirmed Orders will automatically show as Completed when the Restock time has passed. You can see this Completed status in the left column in the Orders section of your RW Elephant account. You can choose to hide all Completed Orders from that column if you'd like.
The Order Status will still show as Confirmed on the Order itself and you cannot choose Completed as an option manually. This is simply a tool to help sort and view Orders in the left column of your Orders section.