Using the Filestar Free plan you get 10 conversion for free every month. You get an e-mail each notifying you when you get new credits.
If you want to use Filestar more than those 10 free conversions per month you can upgrade to the Personal plan. It includes an unlimited number of conversions and costs 9$ annually. You can upgrade by navigating to the 'Help' menu in the app, and then select 'Plans'.