Pay Pal payment process issue

StringBuilder sbPayPal = new StringBuilder();
sbPayPal.Append("https://www.sandbox.paypal.com/cgi-bin/webscr" target="_blank" title="New Window">^]);
sbPayPal.Append("?cmd=_xclick");
// seller account name
sbPayPal.Append("&business=" + "a.hole@webdirekt.com");
sbPayPal.Append("&item_name=" + itemname);
sbPayPal.Append("&amount=" + string.Format(CultureInfo.CreateSpecificCulture("en-US"), "{0:0.00}", price));
sbPayPal.Append("¤cy_code=" +ConfigurationManager.AppSettings["currency_code"]);
sbPayPal.Append("&cancel_return=" + HttpContext.Current.Request.Url);
sbPayPal.Append("&custom=" + customData);
sbPayPal.Append("&first_name=" + userDetailObject.FirstName);
sbPayPal.Append("&last_name=" + userDetailObject.LastName);
sbPayPal.Append("&email=" + userDetailObject.EmailAddress);
sbPayPal.Append("¬ify_url=" + "http://" + HttpContext.Current.Request.Url.Authority + "/popups/mta/callback_proxy.aspx");
sbPayPal.Append("&return=" + "http://" + HttpContext.Current.Request.Url.Authority + "/popups/mta/paymentResponse.aspx");
return sbPayPal.ToString();