Marks a consignment identified by the specified ConsignmentID as printed in the Fastway system.
ConsignmentID - The ID of the consignment to mark all the labels involved as printed. Is often the result of a Add Consignment or
List Consignments call.
UserID - The ID of the customer user who allegedly printed the consignment. If omitted, will default to the one assigned to this API key. A list of ids can be retrieved via the List Users method.
true on success, or an error on failure.