The Application Program Interface is free, we don't ask for any registration fee. You must have a little experience developing software to be able to use it because we do not provide technical support if you don't have a valid premium business subscription (not free).
The API has been widely used to make different software clients and they work well.
Of course any suggestion from our community will be appreciated and we will do our best to satisfy each request. Please send a message to contact the development team.