function validate_form()
{
  valid = true;
  missing = "";
  warning = "#ff0000";
  normal = "#ffffff";

  //
  // Invoice Number
  //
  if (document.paypal.item_number.value == "")
  {
    missing = missing +"\nInvoice Number";
    valid = false;
    document.paypal.item_number.style.backgroundColor = warning;
  }
  else
  {
    document.paypal.item_number.style.backgroundColor = normal;
  }

 

  //
  // Location
  //
  if (document.paypal.amount.value == "")
  {
    missing = missing +"\nAmount to Pay";
    valid = false;
    document.paypal.amount.style.backgroundColor = warning;
  }
  else
  {
    document.paypal.amount.style.backgroundColor = normal;
  }

  if (valid == false)
  {
    alert ("You have not filled out the following information, which is required.\n" + missing);
  }
  else
  {
    document.paypal.item_name.value = "Invoice " + document.paypal.item_number.value + "\n" + document.paypal.item_name.value;
  }
  return valid;
}