public class CalculateShippingDate {

    @InvocableMethod(Label='add two business days')
    public static void addTwoBusinessDaysToCase(List<Case> inputCase){
           Datetime plusTwoDays = BusinessDays.addGmt(System.today(), 2);
           List<Case> caseList = [SELECT Id, Shipping_Date__c
                               FROM Case
                               WHERE Id = :new Map<Id, Case>(inputCase).keySet()];
           Case currentCase = caseList.get(0);
           currentCase.Shipping_Date__c = plusTwoDays.Date();
           update currentCase;

           System.debug('The case was updated with an shipping date date of: ' + currentCase.Shipping_Date__c);
    }
}