The useful thing about Shijusa is that they sell the salt flat in both directions — from Uyuni and from San Pedro de Atacama — which is the choice that actually faces most people doing this trip, and most agencies will only sell you their own end of it.
They publish prices, which in Uyuni is rare: forty-five dollars a head for the full day on the flat, two hundred and fifty for the four-day version that crosses the Sud Lípez to the Chilean border. Those are real numbers to compare a Ferroviaria quote against.
The office in San Pedro is on Caracoles, the one in Uyuni on Ferroviaria. Both are answered on the same Chilean mobile, so budget for an international call if you are not using WhatsApp.