I sold 467 copies over 10 years for less than $5 per copy. It's about $2000 in revenue. Surely not a good use of my time. But I liked the app and so did its few users. We should build things we want to use ourselves right? I think it was a much nicer and faster frontend to the Yelp restaurant directory than the Yelp website.
Your other question about the cost is answered in the blog post and in several other comments I have left in this thread. In short, when I started the app 10 years ago there was no paid API (perhaps there was some kind of "enterprise" version but I'm not sure). The point of my post is how poorly they handled the transition from free->paid.
Yes, definitely based on other people in this thread getting the same letter. It was an inaccurate and threatening form letter bizarrely sent from a personal email address of a Yelp employee with another personal email address CCed.
But how was the API key stored? If it's stored inside the binary, without an intermediate server managed by OP, someone could've disassembled the binary and starting using the API key independently for their own gain, triggering abnormal usage.
Say, one of our Windows products uses a web API for TTS, and we've built an intermediate web server which stores the key and manages rate limits per user so that there was no abuse (the app uses our own auth).
Yelp has an API dashboard where you can see how many API calls have been made. I can tell you it was never exceeding 200 calls per day and often below 100.
Your other question about the cost is answered in the blog post and in several other comments I have left in this thread. In short, when I started the app 10 years ago there was no paid API (perhaps there was some kind of "enterprise" version but I'm not sure). The point of my post is how poorly they handled the transition from free->paid.