Some of the books in our bookstore (most of the PDF versions of e-books) are now Name-Your-Price products. That means that while we set the minimum amount (from $0 to $0.99 in most cases), the amount you actually pay is up to you. Every dollar you add on top of the minimum price is a charitable donation that will be used to keep B. F. Skinner’s books in print, convert more works into e-book formats, provide free access to more and more articles, photographs, videos and other archival material through our website.