public function refundAmount($amount_in_cents, $refund_apply_order = 'credit')
{
$doc = $this->createDocument();
$root = $doc->appendChild($doc->createElement($this->getNodeName()));
$root->appendChild($doc->createElement('refund_apply_order', $refund_apply_order));
$root->appendChild($doc->createElement('amount_in_cents', $amount_in_cents));
return $this->createRefundInvoice($this->renderXML($doc));
}