The best way is to open an issue: https://github.com/LuFlo/texture_atlas_generator/issues
If you purchased via Blender Market, you can also write me a mail.
Good point. You don't have to spend money for this plugin. Consider it as a nice appreciation of the work I put in, also you support the Blender developer fund. It is in our all interest that Blender and its ecosystem grow :-)
I do this in my free time, so I basically improve this add-on when I have time.
This plugin is developed and tested on Blender 2.80 and 2.81 (Linux edition). The migration to 2.7X is not on my high priority list. But any Pull-Requests are welcomed :-)