The ShipStation integration uses the Shipping Method Description and the Carrier (also known as Shipper ID) selected on the Shipping Method. When retrieving the orders, the Shipping Method Description is provided to ShipStation.
This also allows you to use the 3rd-party shipping code, which can be configured in the Shipping Method, with other integrations without additional impact. When the order status is updated, the Shipping Method and Carrier are both returned by ShipStation.
The Shipping Method provided when updating the order status will update the Shipping Method on the order. The Carrier is then used to assign a different Carrier to the tracking number provided so that the Tracking URL provided to the Rep or Customer can be independent of the Carrier on the order's shipping method.
You can verify your current Shipping Methods in Methods and Rates. The carrier selected on the Shipping Method will be listed as Shipper ID on the shipping method.
Methods and Rates: Administration > Settings > Shipping > Methods and Rates.
You can also verify or define additional carriers under Administration > Settings > Shipping > Carriers.
Shipstation does not recognize Parent and Child items on their end, therefore, in order to sort the items on ShipStation's end, the Sort Order on the Inventory Item's configuration is utilized and sent through the Warehouse ID field.
Sort Order: Department > Inventory > Item # > select the Edit pencil > scroll down to find the Sort Order.
For troublshooting any issues, please see the article Troubleshooting ShipStation.