Gridviews, select command + textbox

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ASP.NET

<form id="form1" runat="server">
    <div>

        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
            onselectedindexchanged="GridView1_SelectedIndexChanged">
            <Columns>
                <asp:CommandField ShowSelectButton="True" />
                <asp:BoundField DataField="MethodOfPayment" HeaderText="Method of Payment"
                    SortExpression="MethodOfPayment" />
            </Columns>
        </asp:GridView>
        <br />
        <br />
        <asp:Label ID="Label1" runat="server" Text="Method of Payment:"></asp:Label>
        <asp:TextBox ID="TextBox1" runat="server" AutoPostBack="True"
            ontextchanged="TextBox1_TextChanged"></asp:TextBox>
        <br />
        <asp:Label ID="Label2" runat="server" Text="Installment Amount:"></asp:Label>
        <asp:TextBox ID="TextBox2" runat="server" ontextchanged="TextBox2_TextChanged"></asp:TextBox>

    </div>
    </form>

I have this on my web page. Here are the codes inside.

protected void Page_Load(object sender, EventArgs e)
   {
       if (!Page.IsPostBack)
       {
           SqlConnection cn = new SqlConnection("Connection");
           cn.Open();

           string strSQL;

           strSQL = "SELECT * FROM Orders";

           SqlDataAdapter da = new SqlDataAdapter(strSQL, cn);

           DataSet ds = new DataSet();

           da.Fill(ds, "DB");

           GridView1.DataSource = ds;
           GridView1.DataBind();

           cn.Close();
       }
           Label2.Visible = false;
           TextBox2.Visible = false;

   }
   protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
   {
       TextBox1.Text = GridView1.SelectedRow.Cells[1].Text;
   }
   protected void TextBox1_TextChanged(object sender, EventArgs e)
   {
       if (TextBox1.Text == "Installment")
       {
           Label2.Visible = true;
           TextBox2.Visible = true;
       }
   }

Textbox2 and label2 are supposed to show if the value of the textbox1 is 'Installment'. What did I do wrong? Thank you!