Carbonite has a premium version with a monthly subscription fee. I didn't know addons used to be able to call non visible code. Guess thats how the got away with charging for it.

All add-ons must be distributed free
of charge. Developers may not create "premium" versions of add-ons with
additional for-pay features, charge money to download an add-on, charge
for services related to the add-on, or otherwise require some form of
monetary compensation to download or access an add-on.
The programming code of an add-on must in no way be hidden or
obfuscated, and must be freely accessible to and viewable by the
general public.