I'm trying to create a simple workflow that will set the "Shipping Carrier" or "More" when creating a new sales order. I have set the record type to "Transaction", sub type to "Sales Order", Event "On Create", Trigger Type to "Before Record Load". When i try to create an action to "set field value", I can find the field "Shipping Carrier" on the field list. Anyone have any idea why it is not on the field list? Could something else be happening to this field by another workflow or script?

Do you mean Ship Via aka shipping method?

Another option is you can set the default shipping carrier on a customer page so there would be no need to do it via workflow. Should be able to mass update all the customers fairly quickly.