OK then, i've looked at it and the URL rewriting rule is working fine (Its redirecting to the right page) and the server is clearly running as there is an error document being displayed. I think you may have an error in your PHP script causing the server to be unable to display it. I think we have solved the original problem but stumbled upon another one by mistake. You could test this theory out.
Make a copy of leftproducts.php (Or whatever you are redirecting to) and replace the contents of the origonal with
<?php echo $_GET['c_id']; ?>
(This will output the c_id variable we get from the HTML file)
so
http://yoursite.com/shopping/leftproducts.php?c_id=hello
should output:
hello
Tell me the results of this plz,
Regards,
Sam Rudge