I just spoke with ShipStation, and they informed me that is not currently possible to print the commercial invoice before shipping the order. A suggested workaround was to 'ship' the order, print the documents, and then cancel/void the shipment.
Hopefully being able to print the document without buying a label first will be possible in the future! The customer service agent suggested I post here. Also, being able to download the commercial invoice as an excel file would be a major plus.