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ShipStation provides a setup checklist on their end:
ShipStation Shipping
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Method Service Mapping
You'll also need to make sure that you have mapped your Shipping Methods between the two systems:
What is Service Mapping?
Service Mapping sets the shipping service and package type based on the shipping option your customer chose at time of checkout.
Depending on your store, you may offer a variety of different shipping options to your customers. Common options include Free Shipping, Standard Shipping or Expedited Shipping. ShipStation imports the option the customer chose as the Requested Service.
https://help.shipstation.com/hc/en-us/articles/360025870232-Service-Mapping
https://help.shipstation.com/hc/en-us/articles/360036323651-Use-Shipping-Presets
API integration
Integration uses the Web Service transmission mechanism option in the Transmission mechanism list of the Distribution Center editor .
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