Enabling FedEx One Rate
You can enable FedEx One Rate in ShipHero.
Once you have confirmed you have this feature enabled on your FedEx account (you can do this by contacting your FedEx Account Rep)
Using the top nav, go to Shipping > Carriers, click settings of the FedEx account you want One Rate enabled, and check One Rate.
Once completed, you should see FedEx One Rate as an option when you view shipping quotes on an order.