Where are the code that handle the shipping estimator POST data

I set to show the estimate on shopping cart. When the country was changed, the form POST the data to ...?main_page=shopping_cart. But I was failed to find the codes that handle the POST data. Need help, thanks

If you're using the shipping estimator in-line on the shopping_cart page, that processing is provided by /includes/modules/shipping_estimator.php (or, if you've made template-specific changes, /includes/modules/YOUR_TEMPLATE/shipping_estimator.php).

Thanks, I think it means that handle process is independent from shipping cart. Why I was shown the 404 error when I call this function from other pages? For example. in tpl_product_info_display.php => <?php require(DIR_WS_MODULES . zen_get_module_directory('shipping_estimator.php')); ?>
I had change the $show_in = FILENAME_SHOPPING_CART; to $show_in = FILENAME_PRODUCT_INFO; in shipping_estimator.php