Correct way to pass field data to my script?

class plgHikashopkf_insert extends JPlugin

{

	function plgHikashopKashflow(&$subject, $config){

		parent::__construct($subject, $config);
		if(!isset($this->params)){
			$plugin =& JPluginHelper::getPlugin('hikashop', 'kf_insert');
			jimport('joomla.html.parameter');
			$this->params = new JParameter( $plugin->params );
		}
	}

	function onAfterOrderCreate(&$order,&$send_email){
		
    	if(!empty($order->order_id)){
 		
		$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
		$from = "no_replies_please@********.co.uk";
		$headers .= "From:".$from;
		$email_add = '*******.net';		
		$subject = 'Test';
		
		mysql_connect("localhost", "**********", "************") or die(mysql_error()); 
		mysql_select_db("****************") or die(mysql_error()); 
		
		$order_id = $order->order_id;
		$order_user_id = mysql_result(mysql_query("SELECT order_user_id FROM nl7xh_hikashop_order WHERE '".$order_id."' = order_id ;"), 0);
		$user_email_add = mysql_result(mysql_query("SELECT user_email FROM nl7xh_hikashop_user WHERE '".$order_user_id."' = user_id ;"), 0);
		$order_shipping_address_id = mysql_result(mysql_query("SELECT user_email FROM nl7xh_hikashop_user WHERE '".$order_user_id."' = user_id ;"), 0);
		
		$message = "User ID: $order_user_id Email: $user_email_add;"; //etc
				
        mail($email_add, $subject, $message, $headers);
    	}
		return true;
    }
	
}