def acknowledgment_xml
      xml = Builder::XmlMarkup.new
      xml.instruct!
      @xml = xml.tag!('notification-acknowledgment', {
                        :xmlns => "http://checkout.google.com/schema/2",
                        'serial-number' => serial_number
                      })
      @xml
    end
def acknowledgment_xml
      xml = Builder::XmlMarkup.new
      xml.instruct!
      @xml = xml.tag!('notification-acknowledgment', {
                        :xmlns => "http://checkout.google.com/schema/2",
                        'serial-number' => serial_number
                      })
      @xml
    end
Returns an XML string that can be sent back to Google to
 communicate successful receipt of the notification.