public ActionResult Checkout(CheckoutViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View(model);
            }

            var cart = new ShoppingCartService(HttpContext);

            var result = cart.Checkout(model);

            if (result.Message=="Pass")
            {
                TempData["transactionId"] = result.TransactionId;
                cart.ClearCart(HttpContext);
                return RedirectToAction("Complete");
            }

            ModelState.AddModelError(string.Empty, result.Message);

            return View(model);
        }