ASP.Net RadioButtonList : Input string not correct format

Hello Sir,

I'm quite a beginner , in my project i Found the following error while submiting the form

"Input string was not in a correct format"

Here below my code

public partial class Add_product1 : System.Web.UI.Page
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["DEMO_DbConnectionString"].ToString());
        SqlCommand cmd = new SqlCommand();
        SqlDataReader rdr = null;
        int id = 0, id2 = 0;

 


protected void btnSubmit_Click(object sender, EventArgs e)
        {
            lblError.Text = "";
            try
            {
                cmd.CommandText = "InserttblProduct";
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.Add("@Barcode_No", SqlDbType.Char).Value = txtBarcode.Text;
                cmd.Parameters.Add("@ProductName", SqlDbType.Char).Value = txtNmPro.Text;
                cmd.Parameters.Add("@Volume", SqlDbType.Char).Value = txtVolume.Text;
                cmd.Parameters.Add("@Quantity", SqlDbType.Int).Value = Convert.ToInt32(txtQty.Text);
                cmd.Parameters.Add("@SellingPrice", SqlDbType.Float).Value = Convert.ToDouble(txtSales.Text);
                cmd.Parameters.Add("@Mrp", SqlDbType.Float).Value = Convert.ToDouble(txtMrp.Text);
                cmd.Parameters.Add("@Rate", SqlDbType.Float).Value = Convert.ToDouble(txtRate.Text);
                cmd.Parameters.Add("@Discount", SqlDbType.Float).Value = Convert.ToDouble(txtDiscount.Text);
                cmd.Parameters.Add("@Description", SqlDbType.Char).Value = txtDesc.Text;

                if (txtDiscount.Text != "")
                {
                    cmd.Parameters.Add("@Discount", SqlDbType.Float).Value = Convert.ToDouble(txtDiscount.Text);
                   
                }
                else
                {
                    cmd.Parameters.Add("@Discount", SqlDbType.Float).Value = "0";
                    
                }
                if (txtDesc.Text != "")
                {
                    cmd.Parameters.Add("@Description", SqlDbType.Char).Value = txtDesc.Text;
                    
                }
                else
                {
                    cmd.Parameters.Add("@Description", SqlDbType.Char).Value = null;
                   
                }

                cmd.Parameters.Add("@Active", SqlDbType.Int).Value = Convert.ToInt32(rbtnStatus.SelectedItem.Text);


                con.Open();
                cmd.Connection = con;
                cmd.ExecuteNonQuery();
             
                lblError.Text = "Product Added Successfully.";
              
                txtBarcode.Text = "";
                txtSrNo.Text = "";
                txtNmPro.Text = "";
                txtVolume.Text = "";
                txtQty.Text = "";
                txtSales.Text = "";
                txtMrp.Text = "";
                txtRate.Text = "";
                txtDiscount.Text = "";
                txtDesc.Text = "";
                rbtnStatus.SelectedIndex = 0;
                
            }

            catch (Exception e1)
            {
                lblError.Text = e1.Message.ToString();
            }
            finally
            {
                cmd.Parameters.Clear();
                cmd.Dispose();
                con.Close();

            }
        }

and this is my form design