My plugin is under GPL but I've added this text :
Highslide JS is licensed under a Creative Commons Attribution-NonCommercial 2.5 License: http://creativecommons.org/licenses/by-nc/2.5/
By entering a domain name above and paying the fee, you get the author's permission to use the script under the specified domain name.
The permission is valid for all future upgrades of Highslide JS.
Contact the author: http://vikjavev.no/megsjol/ for group licence or distribution permission.
Highslide site: http://vikjavev.no/highslide/
Is it ok for you?