public void Add(IBillingPlan newPlan)
 {
     Model.BillingPlan model = new Model.BillingPlan()
     {
         Id = newPlan.Id, Calculator = newPlan
     };
     using (IBillingRepository <IBillingPlan> repository = new BillingPlanRepository())
     {
         repository.Add(model);
         repository.SaveChanges();
     }
 }