The custom invoice message could only be set once in the app settings, then applied to every shipment. API integrators had no way to vary it per shipment, which international shipments regularly need.POST /shipments now accepts an optional customs.invoice_message. When present, it replaces the saved message for that shipment only — both on the Ship&co invoice and on the customs declaration sent to the carrier.
Omitting it keeps the current behaviour, and your saved setting is never modified.
The app setting stays available under Settings → invoice settings.
