It is my understanding that Shipstation decided not to pass the shipment value information to Fedex. Therefore when you ship via Fedex using Shipstation and the shipment is lost and the value was over $100, Fedex liability would be limited to $100.
I did talked to Shipstation support about this . They referenced the support article https://www.shipstation.com/docs/api/shipments/get-rates/ stating that this is ALL the information they can get from Fedex. Of course it is a lie, Fedex documentation clearly have declaredValue variable listed - see https://developer.fedex.com/api/en-au/catalog/rate/v1/docs.html
So be ready to lose money if you are using Shipstation and Fedex. That is, unless, of course, you buy the Shipinsurance from Shipstation. Nice way to screw your customers over Shipsurance revenue.