Hi, recently highslide has changed its license, would have to review the features of mAvik as well as within the extension to show the current:

mAvik Features:
add link for pop-up window: Slimbox, Magnific Popup, uikit Lightbox, Highslide (for non-commercial sites - highslide.com/#licence );

Highslide JS is licensed by the MIT-license. This means that you can download and use our software for free, both commercial or non-commercial, as long as you’ve included the MIT-license in all copies or substantial portions of the software

Regards,