Female snails do lay weekly. My blue gives me a clutch every Thursday afternoon, and the wild used to give me one every Sunday morning at about 4 am.
Seriously. Like clockwork.
But yeah, fishyfeet is right, the people who sell clutches like that have tons of snails and get mulitple clutches daily. I had made the mistake of mixing a male into the tank, and I just couldn't bring myself to crush the eggs, so I sold them for postage. Other people do it for some extra cash, and I can't blame them seeing as how much snails eat!