Fancybox popup when shipping method is selected in checkout

Using OC1.5.1.3
When a customer is completing checkout, how can i display another custom php page but in a fancybox popup & when a certain shipping method is selected?
Example:
Shipping methods available are: Pickup & Flat Rate but when a customer ticks the 'Pickup' button, how can I display a another custom php page to them in a fancybox popup?
I have also tried something easier (or so I thought) by using a simple text link in the catalog/language/pickup.php file like this....
<a href="the/path/to/php_file/custom_file.php" class="fancybox">Link Text</a> but for some strange reason, this opens the link in a normal window & not in a fancybox popup.
Further down the checkout & in the 'Payment Method' part, fancybox works when clicking the T&C's link but why does it not work in the link example above?
cheers for any help

Ok, I have managed to get a text link to open a fancybox popup by adding the following to the bottom of the checkout.tpl page.....
<script type="text/javascript"><!--
$('.fancybox').fancybox({
width: 560,
height: 560,
autoDimensions: false
});
//--></script>
but it still would be much better for me if when a customer selects a certain shipping method option, the popup shows although the text link is fine for now