public ActionResult Index(ReceiverViewModel receiver)
 {
     if (ModelState.IsValid)
     {
         //Luu session cho receiver
         SessionHelper.SetReceiverSession(receiver);
         return RedirectToAction("CheckoutByPayPal", "Checkout");
     }
     else
     {
         return View();
     }
 }
 public static void SetReceiverSession(ReceiverViewModel receiver)
 {
     HttpContext.Current.Session["receiver"] = receiver;
 }