The original hypertext transfer protocol—aka http, the protocol that defined the underpinnings of the web—included a code for payments. Everyone knows the 404 error code that pops up when a website isn’t found. But few people know that code 10.4.3 402 was designated as “payment required.” It was “reserved for future use.”
Now that is interesting, and something that I can’t believe still hasn’t been implemented. Think about how many paywalls there are – the New York Times, for example. It’s the perfect use-case.