protected void Calc_Click(object sender, EventArgs e)
        {
            if (IsValid) {
                var receipt = new Model.Receipt(Double.Parse(EditTotal.Text));

                ReceiptPanel.Visible = true;
                SubTotal.Text = String.Format("{0:c}", receipt.Subtotal);
                DiscountRate.Text = String.Format("{0:p0}", receipt.DiscountRate);
                Discount.Text = String.Format("{0:c}", receipt.MoneyOff);
                Total.Text = String.Format("{0:c}", receipt.Total);
            }
        }
        protected void CalculateButton_Click(object sender, EventArgs e)
        {
            if (IsValid)
            {
                Receipt MyReceipt = new Receipt(Double.Parse(AmountTextBox.Text));

                SubtotalLabel.Text = String.Format("{0:c}", MyReceipt.Subtotal);
                DiscountRateLabel.Text = String.Format("{0:p0}", MyReceipt.DiscountRate);
                MoneyOffLabel.Text = String.Format("{0:c}", MyReceipt.MoneyOff);
                TotalLabel.Text = String.Format("{0:c}", MyReceipt.Total);

                ReceiptPanel.Visible = true;
            }
        }