The Field ID of Shipping Cost in a Sales Order record is “shippingcost”. However, when retrieving its value using SuiteScript, the Field ID that should be used is “shippingamount” instead.
The field internal id “shippingamount” is listed under Search Columns.
var lookupFields = search.lookupFields({
type:search.Type.SALES_ORDER,
id: 12345,
columns: ['shippingamount']
})