A simple question but best to ask. To find this file [cscart_dir]/app/addons/paypal/controllers/common/payment_notification.post.php
Its through C-Panel-File Manager-Public_html?
by default (if you have one site in cPanel) - yes
I suppose you better provide more details on what you would like to find out. Now it's not quite clear from you message.
If the root directory of your CS-Cart installation is public_html, then yes