var so = nlapiCreateRecord("salesorder", {entity: customerId});
so.selectNewLineItem("item");
so.setCurrentLineItemValue('item', 'item', itemId);

var a = so.createCurrentLineItemSubrecord("item", "shipaddress");
a.setCurrentLineItemValue("shipaddress", "internalid", shipAddressId);  // yeah this didn't work either
a.commit();
so.commitLineItem("item");