I am not able to find the FedEx SmartPost service under FedEx integration anymore.
This has happened because FedEx SmartPost® service has been renamed to FedEx Ground® Economy. There are some additional options for Indicia Type, which you can edit in Linnworks.net > Shipping > Integrations > FedEx > Services > Edit screen.
You can choose options such as 'Media Mail', 'Bound Printed Matter', 'Returns', 'Under 1lb' and 'Parcel Select'.
FedEx: How to Edit Services