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.
I'm sure your armchair math is much more valid and accurate than their own projections. You should go over there and shower them with your perfect knowledge of the inner workings of their business.
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.