Set up ShipStation multi-carrier rate shopping and implement a custom carrier mapping for a 3PL with non-native carrier accounts

Steps

1. Register the 3PL's carrier accounts in ShipStation via the Carriers API or the Connect Carrier endpoint, providing the account credentials and carrier code
2. Create a ShipStation automation rule that selects the cheapest rate across configured carriers for orders meeting defined criteria (weight range, destination zone, service level)
3. For carriers not natively supported, configure a custom carrier using ShipStation's Custom Carrier integration, mapping the carrier's label API responses to ShipStation's tracking fields
4. Use the ShipStation Rates endpoint to fetch available rates for a shipment before label creation, and select the optimal service programmatically based on business rules
5. Create the label via the ShipStation Labels endpoint with the selected carrier code and service code, then push the resulting tracking number back to the originating platform order via a webhook or direct API call
6. Set up ShipStation webhook subscriptions for ship notification and shipment voided events to keep downstream order management systems synchronized