This is the code in the ShoppingCart.aspx.cs page

 lblTotal.Text = this.FormatPrice(this.Profile.ShoppingCart.Total +
               Convert.ToDecimal(ddlShippingMethods.SelectedValue ));

System.FormatException was unhandled by user code
  Message="Input string was not in a correct format."
  Source="mscorlib"
  StackTrace:
       at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
       at System.Number.ParseDecimal(String value, NumberStyles options, NumberFormatInfo numfmt)
       at System.Decimal.Parse(String s, IFormatProvider provider)
       at System.Convert.ToDecimal(String value)
       at CarStereo.UI.ShoppingCart.UpdateTotals() in c:\Websites\VibeCarStereo\ShoppingCart.aspx.cs:lin e 45
       at CarStereo.UI.ShoppingCart.Page_Load(Object sender, EventArgs e) in c:\Websites\VibeCarStereo\ShoppingCart.aspx.cs:lin e 23
       at System.Web.Util.CalliHelper.EventArgFunctionCaller (IntPtr fp, Object o, Object t, EventArgs e)
       at System.Web.Util.CalliEventHandlerDelegateProxy.Cal lback(Object sender, EventArgs e)
       at System.Web.UI.Control.OnLoad(EventArgs e)
       at System.Web.UI.Control.LoadRecursive()
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Problem solved I had commented out the call to the dropdownlist databind in Page_Load.