^Well, that depends on the distance, doesn't it? If his car gets, say, 20 MPG in the city, then the store would have to be 10 miles away in order for the round trip to expend a full gallon of gas. But he said it was his local store, suggesting something closer. If the store is less than 2.2 miles away, then the gas expense is under 80 cents, and thus the book costs slightly less than it would if purchased online, even with free shipping. And if gas prices fall, it would cost even less. (Not to mention that if it's within a mile or two, you could walk or bike there.)
Plus there are other advantages to visiting a brick-and-mortar store -- the opportunity to browse and stumble across interesting books you wouldn't have found otherwise, the enjoyment of the store's ambience, maybe the relationship with its staff if you're a regular, the support for an endangered institution, etc.