...
Enter a code for the channel partner. Then paste in the username and password values from your ShipStation V1 API key. Once you’ve entered your credentials, click the Refresh button. The Store list will populate with the ShipStation stores if the credentials are correct. Select the correct ShipStation store from the list. Choose skip emails since the marketplace will typically send the confirmation emails to the customer.
...
Shipping Service Mapping
Next, you'll need to specify how UltraCart should map the ShipStation shipping method name to your UltraCart store. UltraCart will perform a shipping method calculation based upon the methods you select below. For example, if you select three available shipping methods for "Standard" (You define this name), UltraCart will calculate which of these methods is available for each ShipStation order that comes through and pick the one that has the top priority and then least expensive one if the priority is not specified. Your preferred method should have a priority of one, then two, etc.
NOTE: You’ll be presented with a configuration and mapping for one shipping service level. To add additional shipping service levels, save the settings then return to the configuration page.
At this point the channel partner is configured. It will poll hourly for new orders and transmit back shipping information as well hourly.