I have tried printing a FedEx shipping label and am seeing an error returned:
8915: COMMERCIAL_INVOICE is required to process your electronic trade document request. (SHIPPING_DOCUMENT=COMMERCIAL_INVOICE)
Commercial Invoice needs to be enabled for some shipping destinations to process the Electronic Trade Document (ETD) request. This can be enabled in Linnworks.net > Shipping > Integrations > FedEx Config > Export Documents section. If ETD is enabled in Fedex Config > Export Documents section, then Commercial Invoice will get sent electronically.