Just a quick update on this topic. This is our item, but we don’t offer custom integration services (like most of authors), so we suggested buyer to ask for paid assistance on Envato forums (because there are always many developers willing to install/customize/tweak items sold by other authors).
In case someone is interested in this job, it will be extremely simple for anyone who has average PHP skills. Auto PHP Licenser offers API which works by making POST requests. The documentation also includes detailed code examples for every single API feature available, and ready-to-use PHP code to make API connection (so users don’t need to create their own).
With this in mind, connecting and sending data to API is as simple as running one line of code. For example, to create a new product, you execute this single line:
Hence, programmer will need to send such POST request(s) from WHMCS (WHMCS supports this feature and offers a full documentation).