When integrating ShipStation to an ERP system, you might want to charge your customer an extra handling fee above and beyond what the carrier is charging. Example a 5% upcharge or a $5 fee per shipment.
ShipStation's checkout rates feature does have the ability to do this, but only for specific store integrations (currently, Shopify, BigCommerce, Magento, and Wix).
We will be expanding the feature to more integrations in the future though. If you would like to see this for a specific ERP or middleware, please let us know which ones in the comments.