Use of PHP Shortcodes in a HTML file

I am using an extension called Order Status Email Templates and like some other modules it makes use of php {shortcodes} to insert things like firstname and lastname etc. These work fine in the emails generated.

Here is the problem, best practice is to add a link to an online version of the email in case of display issues.

Now obviously I can upload a copy of the html to a directory and link to it. However the {shortcodes) will no longer work as they need to be linked in somehow. I have another module called Newsletter Enhancements that manages this and from what I can see has a file in catalog/model/ne/ that handles this called show.php

I guess the other module needs something similar.

Does anyone know a relatively simple way to "tell" a html file what the {shortcodes} mean? Or is it a lot of php development to tie in these?

UIltimately it is up to the php code to have some way to translate your shortcodes.
If you can follow the code as to where the other ones are already defined in the php file, you may be able to just follow those examples to add your own.

Can you advise whether this would be controlled by just one file?

I found a file in Newsletter Enhancements called show.php that appears to control the display of emails when clicked to view in a browser (the url seems to have index.php?roue=ne/show).

show.php is stored in catalog/model/ne/show.php. Is it possible this is the only file that controls these functions? I am not a php guru and only partially understand the MVC concept.