Delivery method

Delivery method

I don't see this returned in SO's details returned in the api so I created a custom field to be updated using %SO_DeliveryMethod%, (which works in templates) but this doesn't work though in the field update. Is this the wrong variable to use / is there another way to get it in the api ?

Update:
It's not returned with a list of sales orders but is when I retrieve a single SO.
I don't really want to make a lot more calls than necessary so does it make sense to update a custom field with this info as all the CF's are returned in the general list ?