Originally Posted by stasys
But why only otf fonts? The most of them are not free.
The best way would be just to include in the extension a disclaimer stating that if somebody will want to sell epubs made with it, then no commercial fonts shall be embedded.
OTF fonts are the only officially supported by ePub. There are a lot of free format converter online that allows to convert all font formati in .otf.
About the disclaimer, it will be not a problem.