public List<SalesDiscount> GetDiscounts()
 {
     return ItemPriceDemoData.GetDiscounts();
 }
 public SalesDiscount GetDiscount(int discountId)
 {
     return ItemPriceDemoData.GetDiscounts().SingleOrDefault(d => d.Id == discountId);
 }
 public SalesItem GetSalesItem(int itemId)
 {
     return ItemPriceDemoData.GetSalesItems().SingleOrDefault(i => i.Id == itemId);
 }