While most of the purchases were made through Amazon, some were made through other digital outlets such as Barnes & Noble, Kobo and Apple itself. (techtimes.com)
The digital PC base version of KILLING FLOOR 2 will be available to purchase for $ 29.99 (US), ₤ 19.99 (UK) and $ 26.99 (EU) via Steam ® and other digital outlets as well as the PC Digital Deluxe version for $ 39.99 (US) / # 29.99 (UK) / $ 39.99 (EU). (pixeljudge.com)
While it requires suspending marketing through other digital outlets (like Nook, Smashwords and eReader), those others were producing very little anyway. (book-marketing-expert.com)