Apparently it's free to master mp3:s. So that would be way to try it out for free, I suppose. For Wav-file mastering you pay monthly for a limited/unlimited number of tracks.
EDIT:
Googled a little bit more and apparently it's been around a few months and a lot of people are highly critical/amused at what Landr claims themselves capable. Still interesting to see if its better than no mastering at all