I've installed P7zip from the Ubuntu Software Center. Under "Optional add-ons" I see a non-free rar module. Is this add-on a trial only? If so, how can I buy the full version?
Files in folders
are licensed under "unRAR license + GNU LGPL" license.
"unRAR license + GNU LGPL" means that you must follow GNU LGPL in all aspects while it is in agreement with unRAR license. But you can not break unRAR license rules. It means that unRAR license is main license in that pair.
The unRAR license is not a free licence, hence the module is non-free. It is freeware, and you don't have to pay anything for using it to its fullest capabilities.