Update.cs

 public int UpdateCartCount(int? id, int cartCount)
        {
            // Get the cart 
            var cartItem = db.CartItem.Single(
                cart => cart.cart_id == ShoppingCartId
                && cart.item_id == id);

            int itemCount = 0;
            //get Promos
            var promos = db.Promotions.Single(promo=>promo.Buy_ItemId==id);

            if (cartItem != null)
            {
                if (cartCount > 0)
                {
                    cartItem.Count = cartCount;
                    itemCount = cartItem.Count;
                    if (cartCount >= promos.BuyX_Qty)
                    {
                        var NewCartItem = new CartItem
                        {
                            item_id = promos.GetYId,
                            product_no = promos.ItemsEntity.product_no,
                            cart_id = ShoppingCartId,
                            Quantity = promos.GetY_Qty,
                            Count = promos.GetY_Qty,
                            Cart_type = "Promotion",
                            DateCreated = DateTime.Now
                        };
                        db.CartItem.Add(NewCartItem);
                    }
                }
                
                else
                {
                    db.CartItem.Remove(cartItem);
                }
                // Save changes 
                db.SaveChanges();
            }
            return itemCount;
        }