I have spoken to support regarding this and their answer was that Royal Mail does not send back a CN22 form along with the label. What would be nice is that when ShipStation identifies that a CN22 is needed (e.g. shipping outside of Europe) that the CN22 form is produced within ShipStation and then tagged on to the next label when printing.
This is URGENTLY required for all UK users of Shipstation that use Royal Mail.
We are having to remember which countries are non-EU members and then manually fill in a CN22 label and attach to parcel.
Netdespath and Metapack automatically know which orders are being shipped to outside EU and print a second label as well as the actual shipping address label
This shouldn't be hard for them, it's just another label to fire out populated from the order details, and would make the standard mail services like Royal Mail, USPS... much more usable for international services.
Due to the lack of this feature we still process our Royal Mail shipments with their click and drop platform and manually copy over tracking details to pass through to aftership. This is very time consuming. If Shipstation added CN22/23 functionality on a 6x4 labels, this would allow Royal Mail international parcels to be processed "under one roof" within Shipstation.