I'd be tempted to replace this
float Amount = float.Parse(Request.QueryString["Amount"].ToString());
with
float Amount = float.Parse(Request.QueryString["Amount"].ToString(), System.Globalization.NumberStyles.Float);
to start with - but be aware, that's likely to fail if the current culture's decimal point separator isn't '.' - so you might need to use this form below instead
float Amount = float.Parse(Request.QueryString["Amount"].ToString(),System.Globalization.NumberStyles.Float, CultureInfo.InvariantCulture);
[edit] btw, giving us the error message might also have been useful [/edit]