I decided to leave out vendor-specific stores, and I did omit some other sources, such as Amazon and GamersGate. (And the callous reality is that I could have written all day about different stores, but I wouldn't have earned any more money for the post.)
If the topic proves popular enough with readers I may revisit it and talk about some of the other options. GOG, Steam, and Humble worked well for the story because each has a distinctly different niche and operating model.