An Order can be closed once all items on the Order have been checked back in, the Outstanding Balance is $0.00, and there are no remaining Deposits on the Order.
In order to clear an Outstanding Balance, you can create an Invoice or Refund for everything that is on the order and Refund any Deposits that remain (if applicable)
The Invoices do not need to be Paid in order to Close the Order. A Closed Order just indicates that the rental is completed and everything has been billed for.