The Itch.io version of Jav-It is an older release that provides basic scraping functionality. It previously supported R18 decryption as well, but that functionality has since been deprecated following the shutdown of that service.
For advanced scraping features, including subtitles, DMM and VR decryption, and other functionality, you need to be an active Patreon supporter. Patreon supporters receive regular updates, access to our Discord community, and unlimited access to our community servers, which handle the heavy lifting behind the scenes. Jav-It is essentially a thin client, and requires access to our community cloud servers to work.
As for purchases, decryption only works for videos which allow you to download them. As far as I know, one-time rentals only allow streaming, which we no longer support due to the unreliable nature of it all.