Thanks for writing in. While we can't currently print a label for a shipment that you have not bought postage for, you can use the "Mark as Shipped" function on the Orders page for that item. You can then leave a note in the "Tracking" area that will be visible on your Shipment History page.
This will not give you a printed label, but it will update your store that the item is shipped and give you information that can be checked within ShippingEasy. If you need a print out with the customers address, would a packing slip work?
I hope this helps, please let me know if there are any specifics that this process does not address.