Used to indicate the third-party billing account associated with the shipment. Each account is tied to a specific carrier and will only be used if the associated carrier is selected for label creation. If a selected carrier does not have a corresponding third-party billing account in this list and lacks a configuration in the fulfillment context, third-party billing will not be applied for that label. Conversely, if a third-party billing configuration exists in the fulfillment context and this value is provided, the value specified here will take precedence over the configured value.