I’m Trying to use event textbox change but I have an error “Input string was not in a correct format “

Is there something wrong with my code ?

or

Is there something wrong with my code implementation ?

Please Guide me

Thanks

Public Class Payment
    Private salesservices As New SalesService()
    Private Sub Payment_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim sales = salesservices.GetSales(LblInvono.Text)
        LabelTotalvalue.Text = sales.Totalinv.ToString("N0")
        txtDiscount.Text = sales.DiscountTotal.ToString("N0")
        txtPayment.Text = (sales.CashPayment + sales.NCashPayment).ToString("N0")
        LabelChange.Text = ((sales.CashPayment + sales.NCashPayment + sales.DiscountTotal) - sales.Totalinv).ToString("N0")
    End Sub
Private Sub OnTextChanged(sender As Object, e As EventArgs) Handles txtPayment.TextChanged, txtDiscount.TextChanged
        Dim payment As Double = 0
        Dim discount As Double = 0
        If Not String.IsNullOrEmpty(txtPayment.Text) OrElse Not String.IsNullOrEmpty(txtDiscount.Text) Then
           'Error codes below this code line  
           payment = Convert.ToDouble(txtPayment.Text)
            discount = Convert.ToDouble(txtDiscount.Text)
            LabelChange.Text = (Convert.ToDouble(LabelTotalvalue.Text) - discount - payment).ToString("N0")
        Else
            LabelChange.Text = "-" & Convert.ToDouble(LabelTotalvalue.Text).ToString("N0")
        End If
    End Sub
    Private Sub OnText_KeyPress(sender As Object, e As KeyPressEventArgs) Handles txtPayment.KeyPress, txtDiscount.KeyPress
        If Not Char.IsControl(e.KeyChar) AndAlso Not Char.IsDigit(e.KeyChar) Then
            e.Handled = True
        End If
    End Sub
    Private Sub txtPayment_KeyUp(sender As Object, e As KeyEventArgs) Handles txtPayment.KeyUp
        txtPayment.Text = txtPayment.Text.Replace(",", "")
        Dim discount As Double = 0
        Dim payment As Double = 0
        If Not String.IsNullOrEmpty(txtPayment.Text) Then
            payment = Convert.ToDouble(txtPayment.Text)
            LabelChange.Text = (Convert.ToDouble(LabelTotalvalue.Text) - discount - payment).ToString("N0")
            txtPayment.Text = Convert.ToDouble(txtPayment.Text).ToString("N0")
            txtPayment.[Select](txtPayment.Text.Length, 0)
        End If
    End Sub
    Private Sub txtDiscount_KeyUp(sender As Object, e As KeyEventArgs) Handles txtDiscount.KeyUp
        txtDiscount.Text = txtPayment.Text.Replace(",", "")
        Dim discount As Double = 0
        Dim payment As Double = 0
        If Not String.IsNullOrEmpty(txtPayment.Text) Then
            discount = Convert.ToDouble(txtDiscount.Text)
            LabelChange.Text = (Convert.ToDouble(LabelTotalvalue.Text) - discount - Payment).ToString("N0")
            txtDiscount.Text = Convert.ToDouble(txtDiscount.Text).ToString("N0")
            txtDiscount.[Select](txtPayment.Text.Length, 0)
        End If
    End Sub
End Class