C# รบกวน ช่วยทีคะเวลาคลิกเลือกรายการใน listbox อยากให้มันรวมราคาใว้ใน label อะคะ

using System.Data;
using System.Data.SqlClient;
using System.Configuration;
namespace Book
{
    public partial class sellbook : System.Web.UI.Page
    {
        string cn = ConfigurationManager.ConnectionStrings["sqlCon"].ToString();
        DataSet ds = new DataSet();
        SqlDataAdapter da;

        protected void Page_Load(object sender, EventArgs e)
        {

            if (!Page.IsPostBack)
            {
                lbltype.Text = Session["TName"].ToString();

                string sql = "select * from Book";
                SqlDataAdapter da = new SqlDataAdapter(sql, cn);
                da.Fill(ds, "Book");

                ListBox1.DataSource = ds.Tables["Book"];
                ListBox1.DataTextField = "BNAME";
                ListBox1.DataValueField = "BPrice";
                ListBox1.DataBind();
            }
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            Session["username"] = lbluser.Text;

            Response.Redirect("count.aspx");
        }

        protected void Button2_Click(object sender, EventArgs e)
        {
            txtname.Text = ListBox1.SelectedItem.Text;
        }

        
        int total;
        int price;

        protected void Button3_Click(object sender, EventArgs e)
        { 
            ListBox2.Items.Add(txtname.Text + " " + "จำนวน" +" "+ txtunit.Text + " " + "เล่ม");

            string sub = "";
            int unit = Convert.ToInt16(txtunit.Text);
            int unitprice = Convert.ToInt16(ListBox1.SelectedValue);
            int count = ListBox1.Items.Count;
            
            for (int i = 0; i < count; i++)
            {
                if (ListBox1.Items[i].Selected)
                {
                    sub += ListBox1.Items[i].Text + " ";
                    price = unit * unitprice;       
                    total += price;
                }
           }

            lblprice.Text = total.ToString();

        }

     }
}