serviceCode is available on the DeliveryMethod API
API
Effective October 01, 2023
As of the { 2023-10 } API version, you can use { DeliveryMethod } to get the value of the { serviceCode } chosen for the related { FulfillmentOrder} . The { DeliveryMethod } object is now accessible through any of the fulfillment order scopes.
Learn more about { DeliveryMethod } on Shopify.dev.