API requires only transmission of FTP data
The plugin should be managed by the reseller (or admin) and can be enabled and deactivated per user by reseller as addon.
To register with SitePro you need to select the premium version, there is a bug with free version signup, it does not work.
If you are registered you will not be charged, It is only the access to the license management and API implementation details.
There are already plugins for C-Panel, Plesk, ISP etc. and a PHP API sample is available, could may be used as a model for a sitepro imSCP plugin.