crm simple c# plugin development

I need to develop a plugin in c# when the zipcode is 12345 the freight amount in quote is set to $100.

How to retrieve the ship to zip code from crm like this is done in another logic i found:

Entity QuoteDetail = orgService.Retrieve("quote", quoteId, new ColumnSet(new String[] { "freightamount", "shipto_stateorprovince", "totalamountlessfreight", "imc_taxexempt" }));
I cannot retrieve the shipto_zipcode .