If you are needing to bill your customer's account you can use the following instructions from our Third-Party Billing Guide on our help center.
Basically, here is what you do:
Set the shipping service and package type for the order.
Since ShipStation requires you to first select the correct service, this means you must already have your own carrier account connected to ShipStation.
For example, to bill to a recipient's UPS account, you must have your own UPS account connected to ShipStation, then select a UPS service for that order.
Go to Other Shipping Options in the Shipping Sidebar and click the Shipping Account drop-down.
ShipStation Other Shipping Options:
ShipStation Legacy Other Shipping Options:
Choose either Third Party or Recipient from the list.
Enter the third party Account #, Billing Country, and Billing Zip (Postal Code).
Create the label.
And that should do it!