This is what I have now. But it will only get the latest update of the arraylist.

Description page:

ArrayList idList = new ArrayList();

            idList.Add(ProdID);
            idList.Add(ProQty);
            Session["InstallationId"] = idList;
            Response.Write("<script>if(confirm('Added to cart!')){} else { return false; }</script>");

Check out page:

ArrayList idList = (ArrayList)Session["InstallationId"];
        for (int index = 0; index < idList.Count; index++)
        {
            Label1.Text = Label1.Text + idList[index].ToString();
        }