There is an error returned when printing a FedEx label:
The COMMERCIAL_INVOICE requires an electronic LETTER_HEAD. (SHIPPING_DOCUMENT=COMMERCIAL_INVOICE IMAGE_USAGE=LETTER_HEAD)
The electronic letter-head and signature are required for some destinations when shipping via FedEx and can be added to the FedEx config on your account.
You will need to create the images and copy them to a file sharing service such as Dropbox, Google Drive or an FTP server. For instructions on how to get the links, see this article.
Note that the maximum size is 700 x 50 pixels for the letterhead and 240 x 25 pixels for the signature. The links to these files need to be entered into the image URL fields under Document Options in the integration config (Linnworks.net > Shipping > Integrations > FedEx > Config).
An example of the letter-head and signature would look like this: