I find it a little shady to wait for people to get to half the course before requiring to buy it.
Edit: after actually using the site I see your point, the only indicator is the small text "free preview" which I probably wouldn't have noticed without first having read the Show HN post.
As distasteful as it may be, an actual heading or otherwise strongly formatted text note near the top should suffice for most people.
Honestly, you should just be giving away this whole chapter. If you did then I could forward it to colleagues, let them learn this one concept and suggest they get the rest of the info.
As it is I really can't do that. It'd be like giving a friend a flyer that was shoved under my door.