I'm not sure why that isn't working, It works fine on my test. But here is an alternative technique that might help.
Instead of calling the method in the AJAX url, just use the page .aspx url, and add the method as a parameter in the data object. Then when it calls page_load, your data will be in the Request.Form variable.
jQuery
jQuery.ajax({
url: 'AddToCart.aspx',
type: "POST",
data: {
method: 'AddTo_Cart', quantity: total_qty, itemId: itemId
},
dataType: "json",
beforeSend: function () {
alert("Start!!! ");
},
success: function (data) {
alert("a");
},
failure: function (msg) { alert("Sorry!!! "); }
});
C# Page Load:
if (!Page.IsPostBack)
{
if (Request.Form["method"] == "AddTo_Cart")
{
int q, id;
int.TryParse(Request.Form["quantity"], out q);
int.TryParse(Request.Form["itemId"], out id);
AddTo_Cart(q,id);
}
}