How impactful is Costco on new book sales?

I was just there today and hardcover books were flying off the table like candy (at $15 each), which got my thinking about the impact of the big box retailers on new book sales.  It used to be that people would reserve new copies at bookstores or even pre-ordered online, but now, for most big authors, you can just pop into Costco and pick up a new hardcover at pretty good price.

Additionally, I considered the fact that there is limited book real estate in each Costco store, which leads to questions about what authors get table space.  The well-known authors (i.e., Tom Clancy) will always get table space, but I wonder if publishing houses actually pay for table space at Costco for authors that aren’t quite as well known.