Relay Response URL is used to return the customer to invoice page after successful payment. Authorize.Net may issue the <b>Error 14</b> (The Referrer or Relay Response URL is invalid) in case this URL is not present in the <b>Merchant Interface</b>. <br/> Read more at the <a href='https://support.authorize.net/authkb/index?page=content&id=A601' target='_blank'>Authorize.Net Support Center</a>