When exporting items with Parcelforce a commercial invoice is required. This will be generated automatically if automatic printing has been enabled within Shiptheory.
When using Parcelforce a Parcelforce specific commercial invoice is generated, by Parcelforce, using the commodity data that you have provided.
Note: if you have selected the Enable documents only option from the advanced settings section of the Parcelforce settings page, the shipment will be treated as documents only and no commercial invoice will be printed as it is not required.
To enable automatic printing of commercial invoices, please follow this guide here: http://support.shiptheory.com/support/solutions/articles/24000044841