code: protected function getTransactionReference( \Aimeos\MShop\Order\Item\Base\Iface $base )
  	{
  		$code = $this->getServiceItem()->getCode();
  		$service = $base->getService( \Aimeos\MShop\Order\Item\Base\Service\Base::TYPE_PAYMENT, $code );
  
  		return $service->getAttribute( 'TRANSACTIONID', 'payment/omnipay' );
  	}
docs: Returns the payment transaction ID stored in the basket
  
  @param \Aimeos\MShop\Order\Item\Base\Iface $base Basket including (payment) service items
  @return string|null Payment transaction ID or null if not available