title: I keep getting An exception of type 'System.InvalidCastException' occurred in mscorlib.dll but was not handled in user code for RetailPrice
body: <pre><code>protected void btnCalculateOrder_Click(object sender, EventArgs e)
  {
      OrderSession();
  
      if (IsValid)
      {
          double RetailPrice = Convert.ToDouble(lblRetailPrice);
          double Quantity = Convert.ToDouble(txtQuantity);
          double Tax = (.9);
  
          double Subtotal = RetailPrice * Quantity;
          double TotalAmount = Subtotal + Tax;
  
          lblSubTotal.Text = Subtotal.ToString("c");
          lblTotalAmount.Text = TotalAmount.ToString("c");
      }
  
  }
  </code></pre>