trigger billingSubscriptionEmail on Opportunity (before update) {
if(BillingEmailTriggerHandler.isFirstTime)
{
BillingEmailTriggerHandler.isFirstTime = false;

List<Addon_Opportunity__c> addOppList = new List<Addon_Opportunity__c>([SELECT Name,Amount__c,Quantity__c FROM Addon_Opportunity__c where Id__c IN :Trigger.newMap.keySet()]); 

List<Contact> conList = new List<Contact>([select Email from Contact where Id In (select ContactId from OpportunityContactRole where OpportunityId IN :Trigger.newMap.keySet())]); 

 Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
  
    for(Opportunity opp : trigger.new){
    
            if(opp.StageName=='Closed Won' && Trigger.oldMap.get(opp.Id).StageName!='Closed Won'){
                
                for(Contact c:conList){
               
                 String[] toAddresses = new String[] {'ahmeds@webgility.com'};
                 String billingCycle;
                 String promo='';
                   
                    mail.setToAddresses(toAddresses );   
                    mail.setSubject('New Subscription - '+opp.Accounting_Platform__c+' - '+c.Email);   
                   if(opp.Billing_Cycle__c=='Monthly'){
                     billingCycle = 'Monthly';
                   }else{
                      billingCycle = 'Annual';  
                   }
                   
                  if(opp.promo_code__c!=null){
                      promo = opp.promo_code__c;
                  }else{
                      promo = 'None';
                  }
                    String template = 'Billing & Subscription Info:\n\n';
                     template+= '----------------------------------\n';
                     template+= billingCycle+' Recurring Amount: $'+opp.Recurring_Revenue__c+'\n';
                     template+= 'Plan: '+opp.Plan__c+'\n';
                     template+= 'Plan Tier: '+opp.Plan_Tier__c+'\n';
                     template+= 'Account Email: '+c.Email+'\n';
                     template+= 'Total Upfront Payment: $'+ opp.One_Time_Fee__c+'\n'; 
                     template+= '----------------------------------\n';
                     for(Addon_Opportunity__c addon :addOppList){
                        
                         template+= addon.Quantity__c+' '+addon.Name+' : $'+addon.Amount__c;
                         template+= '\n';
                         
                    }
                    template+= '\n';
                    template+= 'One Time Fee: $'+opp.One_Time_Fee__c+'\n';
                    template+= '----------------------------------\n';
                    template+= 'Recurring billing start date :'+opp.Purchase_Date__c+'\n';
                    template+= 'Promo Code :'+promo;
                    List<String> args = new List<String>();
               String formattedHtml = String.format(template, args);
               
                mail.setPlainTextBody(formattedHtml);
                
              }
              Messaging.SendEmail(new Messaging.SingleEmailMessage[] {mail});  
           }   

    }

 }
}

@isTest()
public class TestbillingSubscriptionEmail {

    public static testMethod void TestbillingSubscriptionEmailTest()
    {
       
         Account testAcct = new Account (Name = 'TestbillingSubscriptionEmail');
		insert testAcct;
        Contact con = new Contact(LastName ='Kamran',Email='ahmeds+18augtest13@webgility.com', AccountId = testAcct.ID);
        insert con;
		  
		Opportunity opp = new Opportunity(Name='test opp', StageName='stage', Probability = 95, CloseDate=system.today());
		insert opp;
		  
		  //Create the opporunityContactRole.
		  opportunityCOntactRole oppCOn = new opportunityCOntactRole(OpportunityId=opp.id, contactId= con.Id, isPrimary=true);
		  insert oppCon;
		  
		  //update the opportunity so that it is closed and enters the if conditon in your trigger.
		  Test.StartTest();
		  
			opp.stageName='Closed Won';
			update opp; 
		  Test.StopTest();
    
    }
}