Jquery - Set Session In PageMethod?

I need to set a couple of Session vars by calling a PageMethod using jQuery.

The client side js looks like this:

function setSession(Amount, Item_nr) {
//alert(Amount + " " + Item_nr);
var args = {
amount: Amount, item_nr: Item_nr
}
//alert(JSON.stringify(passingArguments));
$.ajax({
type: "POST",
url: "buycredit.aspx/SetSession",
data: JSON.stringify(args),
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function () {
alert('Success.');
},
error: function () {
alert("Fail");
}
});
}

and the server side like this:

[System.Web.Services.WebMethod(EnableSession = true)]
public static void SetSession(int amount, int item_nr)
{
HttpContext.Current.Session["amount"] = amount;
HttpContext.Current.Session["item_nr"] = item_nr;
}

only, it seems that the Session vars are not set. When I try to Response.Write out the Session vars, I get nothing. I get no errors, and I can alert out the values passed from the onclick event, to the js function, so they are there.