From ISC doc:
It’s also possible to include another file within a template. This can be accomplished using file include variables. This is extremely useful if external files such as banners, site‐network headers or other content need to be included. File includes can either be local (relative to the Interspire Shopping Cart base directory), contain absolute file system paths, or be remote includes by specifying the full URL to a file to be included.
The local file include is not working due to a coding bug in /public_html/lib/templates/template.php
//$includeRoot = ISC_BASE_PATH . '/templates/__includes/'; $includeRoot = ISC_BASE_PATH . '/';
Now we can include any local files in templates