A small Ferroviaria operator — the website, the email address and the WhatsApp account are all in Beimar Villa's name, which is honest about what you are buying: a driver-guide and a Landcruiser rather than an agency with a rota.
That is not a criticism in Uyuni. The difference between a good three days and a bad one is which driver you get, and booking directly with one who answers his own phone is a defensible way to choose.