Here is how you can use the URL key for a product, using getUrl and avoid the trailing slash
If you use
$box_url = Mage::getUrl('our-box.html');
and echo that string you will get something like
http://whatever.com/our-box.html/
That of course is not right and you probably will get a 404 page. Unless you did a URL rewrite but that is another topic all together. Here is how to use the same function, but use a parameter instead
$box_url = Mage::getUrl('', array('_direct'=>'our-box.html'));
That will get us what is needed so echoing this string gets us
http://whatever.com/our-box.html