protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                // Verify user has completed the checkout process.
                if ((string)Session["userCheckoutCompleted"] != "true")
                {
                    Session["userCheckoutCompleted"] = string.Empty;
                    Response.Redirect("CheckoutError.aspx?" + "Desc=Unvalidated%20Checkout.");
                }

                PayPalApi payPalCaller = new PayPalApi();

                string retMsg = "";
                string token = "";
                string finalPaymentAmount = "";
                string PayerID = "";
                NVPCodec decoder = new NVPCodec();

                token = Session["token"].ToString();
                PayerID = Session["payerId"].ToString();
                finalPaymentAmount = Session["payment_amt"].ToString();

                bool ret = payPalCaller.DoCheckoutPayment(finalPaymentAmount, token, PayerID, ref decoder, ref retMsg);
                if (ret)
                {
                    // Retrieve PayPal confirmation value.
                    string PaymentConfirmation = decoder["PAYMENTINFO_0_TRANSACTIONID"].ToString();

                    // Clear order id.
                    Session["currentOrderId"] = string.Empty;
                }
                else
                {
                    Response.Redirect("CheckoutError.aspx?" + retMsg);
                }
            }
        }
        private string buildCredentialsNVPString()
        {
            NVPCodec codec = new NVPCodec();

            if (!IsEmpty(APIUsername))
                codec["USER"] = APIUsername;

            if (!IsEmpty(APIPassword))
                codec[PWD] = APIPassword;

            if (!IsEmpty(APISignature))
                codec[SIGNATURE] = APISignature;

            if (!IsEmpty(Subject))
                codec["SUBJECT"] = Subject;

            codec["VERSION"] = "88.0";

            return codec.Encode();
        }
        public bool GetCheckoutDetails(string token, ref string PayerID, ref NVPCodec decoder, ref string retMsg)
        {
            if (bSandbox)
            {
                pEndPointURL = pEndPointURL_SB;
            }

            NVPCodec encoder = new NVPCodec();
            encoder["METHOD"] = "GetExpressCheckoutDetails";
            encoder["TOKEN"] = token;

            string pStrrequestforNvp = encoder.Encode();
            string pStresponsenvp = HttpCall(pStrrequestforNvp);

            decoder = new NVPCodec();
            decoder.Decode(pStresponsenvp);

            string strAck = decoder["ACK"].ToLower();
            if (strAck != null && (strAck == "success" || strAck == "successwithwarning"))
            {
                PayerID = decoder["PAYERID"];
                return true;
            }
            else
            {
                retMsg = "ErrorCode=" + decoder["L_ERRORCODE0"] + "&" +
                         "Desc=" + decoder["L_SHORTMESSAGE0"] + "&" +
                         "Desc2=" + decoder["L_LONGMESSAGE0"];

                return false;
            }
        }
        public bool ShortcutExpressCheckout(string amt, ref string token, ref string retMsg)
        {
            if (bSandbox)
            {
                pEndPointURL = pEndPointURL_SB;
                host = host_SB;
            }

            string returnURL = "http://localhost:50298/User/CheckoutReview.aspx";
            string cancelURL = "http://localhost:50298/User/CheckoutCancel.aspx";

            NVPCodec encoder = new NVPCodec();
            encoder["METHOD"] = "SetExpressCheckout";
            encoder["RETURNURL"] = returnURL;
            encoder["CANCELURL"] = cancelURL;
            encoder["BRANDNAME"] = "Nelo Lodging";
            encoder["PAYMENTREQUEST_0_AMT"] = amt;
            encoder["PAYMENTREQUEST_0_ITEMAMT"] = amt;
            encoder["PAYMENTREQUEST_0_PAYMENTACTION"] = "Sale";
            encoder["PAYMENTREQUEST_0_CURRENCYCODE"] = "USD";

            string pStrrequestforNvp = encoder.Encode();
            string pStresponsenvp = HttpCall(pStrrequestforNvp);

            NVPCodec decoder = new NVPCodec();
            decoder.Decode(pStresponsenvp);

            string strAck = decoder["ACK"].ToLower();
            if (strAck != null && (strAck == "success" || strAck == "successwithwarning"))
            {
                token = decoder["TOKEN"];
                string ECURL = "https://" + host + "/cgi-bin/webscr?cmd=_express-checkout" + "&token=" + token;
                retMsg = ECURL;
                return true;
            }
            else
            {
                retMsg = "ErrorCode=" + decoder["L_ERRORCODE0"] + "&" +
                         "Desc=" + decoder["L_SHORTMESSAGE0"] + "&" +
                         "Desc2=" + decoder["L_LONGMESSAGE0"];
                return false;
            }
        }
        public bool DoCheckoutPayment(string finalPaymentAmount, string token, string PayerID, ref NVPCodec decoder,
            ref string retMsg)
        {
            if (bSandbox)
            {
                pEndPointURL = pEndPointURL_SB;
            }

            NVPCodec encoder = new NVPCodec();
            encoder["METHOD"] = "DoExpressCheckoutPayment";
            encoder["TOKEN"] = token;
            encoder["PAYERID"] = PayerID;
            encoder["PAYMENTREQUEST_0_AMT"] = finalPaymentAmount;
            encoder["PAYMENTREQUEST_0_CURRENCYCODE"] = "USD";
            encoder["PAYMENTREQUEST_0_PAYMENTACTION"] = "Sale";

            string pStrrequestforNvp = encoder.Encode();
            string pStresponsenvp = HttpCall(pStrrequestforNvp);

            decoder = new NVPCodec();
            decoder.Decode(pStresponsenvp);

            string strAck = decoder["ACK"].ToLower();
            if (strAck != null && (strAck == "success" || strAck == "successwithwarning"))
            {
                return true;
            }
            else
            {
                retMsg = "ErrorCode=" + decoder["L_ERRORCODE0"] + "&" +
                         "Desc=" + decoder["L_SHORTMESSAGE0"] + "&" +
                         "Desc2=" + decoder["L_LONGMESSAGE0"];

                return false;
            }
        }