My name is Emma and I'm one of the Support Gurus. ShipStation is a pretty popular app and is great for a few different reasons:
1) It offers discounted rates/labels
2) You can print labels for different services such as USPS, FedEx ect
3) You can do batch labels rather then having to do each order individually
4) ShipStation also communicates the shipping/tracking information back to Shopify so that it can be shared in the notification email to the customer
Manually entering shipping information into FedEx website is cumbersome. You have to copy paste every little order info manually and this takes both time and money. To overcome this, you are trying to use Shipstation. I would recommend another app - Multi carrier Shipping label app that saves both time and money. Using the app you can show real time shipping rates at checkout, generate labels in bulk, generate manifest, request pickups and track shipments in real time. This is completely automatic so the order info is sent to FedEx without you having to manually put in every detail.