We tried loading shipping item data with 'nlapiLoadRecord', Currently we have two methods available Standard & Express. We tried to load data for standard method as below in Script Debugger.
var standard = nlapiLoadRecord('shipitem',3);
We are getting all fields of standard shipitem in Script Debugger, then we are fetching shipping rule value with
var standard_amount = standard.getFieldValues('freeifordertotalisoveramount');
Basically this is not case with your code, as you are trying to load shipping rate using SuiteScript Service and model this is perfect.
var standard = nlapiLoadRecord('shipitem',3);
var standard_amount = standard.getFieldValues('freeifordertotalisoveramount');
This is completely related with user's permission i.e Roles Under Setup-->User/Roles-->Manage Roles you are trying to fetch data of 'shippingitem' in this case you must need to check different permission
Most Imp: 2. Check shipping item is added in permission under List i.e the role which you are using for Service, for that role you must need to set shipping item See below screen shot you will get more idea.