When we've run into this issue it's usually the Ship From location not being setup to pull from the correct warehouse, or the Ship From location not being set correctly.
Your screen shot doesn't show your Ship From location, but have you confirmed that is linked up correctly? You should be able to go to Shipstation settings, Shipping > Ship From Locations and select your location. The last setting on your location should be 'Inventory Source'. Based on your screen shot, your inventory source should be set to 'Fulfillment Center'. Is that setting set correctly?
When you're viewing your order, is your Ship From location being populated on its own?
