I've never heard of this site before, but looking at their page 'How it works', it seems clear it me it's a shoe-of-the-month club, and works similar to things like Disney Movie club, Harry and David Fruit of the month club, or one of various Book Clubs.

The only scummy part would be if they still charge $39.99/month if you don't cancel / skip / select a shoe, without sending you anything. Compare to Disney Movie of the month club where if you don't make a selection or skip that month, you get sent the "Featured" title for that month. Maybe justfab should send you the shoe of the month if you don't make a selection, or maybe they should send a "beauty gift basket".

Otherwise, it looks legit, if you like this sort of thing.