Navigate to the Rules page from the lefthand sidebar and select Shipping Boxes from the available headers. Next click the Create a Rule button:
Following "If..." on the resulting righthand sidebar, you'll see three parameters our system will ask you to provide for your rule to be enforced:
Next, assign the shipping box(es) you wish to assign to orders that meet the parameters you've provided above by selecting them from the dropdown under Select Shipping Boxes then click the Create Rule button to finalize setup:
In the steps outlined in this example, we've created the rule "If an order is going to a customer in TX, use our 'Texas Promo Shipper' Shipping Box to ship it".