code: protected function saveTransationRef( \Aimeos\MShop\Order\Item\Base\Iface $baseItem, $ref )
  	{
  		$type = \Aimeos\MShop\Order\Item\Base\Service\Base::TYPE_PAYMENT;
  		$serviceItem = $this->getBasketService( $baseItem, $type, $this->getServiceItem()->getCode() );
  
  		$attr = array( 'TRANSACTIONID' => $ref );
  		$this->setAttributes( $serviceItem, $attr, 'payment/omnipay' );
  		$this->saveOrderBase( $baseItem );
  	}
docs: Adds the transation reference to the order service attributes.
  
  @param \Aimeos\MShop\Order\Item\Base\Iface $baseItem Order base object with service items attached
  @param string $ref Transaction reference from the payment gateway