private void TXT_QUA_KeyDown(object sender, KeyEventArgs e)
{
if (e.KeyCode == Keys.Enter && TXT_QUA.Text != string.Empty)
{
TXT_DISCOUNT.Focus();
}
}
void CLEAR_TXTS()
{
TXT_ID_PRODUCT.Clear();
TXT_NAME_PRODUCT.Clear();
TXT_PRICE.Clear();
TXT_QUA.Clear() ;
TXT_AMOUNT.Clear();
TXT_DISCOUNT.Clear();
TXT_TO_AMOUNT.Clear();
}