ShipMethod (again and final).
I have to say that I got a lot of help from Netsuite Support 🙂
To get the shipmethod text and id, the code is
SELECT transactionshipment.shippingmethod, shipitem.id FROM transactionshipment JOIN transaction on transaction.id=transactionshipment.doc JOIN shipitem ON transactionshipment.shippingmethod=shipitem.displayname where transaction.id = 40711367