8 – How can I transition an order from fulfillment to completed?

I’m running Commerce Marketplace and provide store owners a back-end portal for managing their store.

In their portal, they can view orders sitting in the “fulfillment” state with the shipping address. Once they fulfill that order, I need to provide a button for them to move that order to the “completed” state.

How do I move an order to the “completed” state? Do I send the user to a specific checkout route, or can I move the order itself to completed via the API?