Typically you get 2 weeks of vacation a year. That's 4% of the cost. Not a significant amount.

As for work outside store hours: sure, there is some. But it's not a restaurant where you have to sterilize everything in the kitchen. It's a bookstore. At most you have to sweep the floors, maybe re-shelf some misplaced books, and shelf the newcomers. It's not a tremendous amount of work that has to be done before opening or after close.