Asking if some action was politically motivated will not yield an answer you can trust. A better way to determine if it was politically is to review a list of the books Costco does offer at the same time.
One Person; One store.
The largest book display was for Hillary Clinton's "Hard Choices." I counted 462 copies and about 24 CD audio sets.
Patrick J. Buchanan’s "The Greatest Comeback: How Richard Nixon Rose from Defeat to Create the New Majority" was about 50 copies.
Dr. Ben Carson's "One Nation: What We Can All Do to Save America's Future" was around 80 copies.
Edward Klein's "Blood Feud: The Clintons vs. the Obamas” was about 80 copies.
Daniel Schulman's "Sons of Wichita: How the Koch Brothers Became America's Most Powerful and Private Dynasty" was about 10 copies.
This set of data is insufficient to draw a conclusion. However, more reports like this would provide enough data.