Rip off of what — an existing tool, or the price? Genuinely want to know which so I can actually answer.
5 bucks for a single QR.
Fair. To be clear: the engine is open source (sqrart/qart on Packagist / GitHub) — you can generate these for free and self-host. The €5 pays for the hosted side: a validated render, a destination you can change later without reprinting, and scan stats. If you just want the image once, the library does that for nothing.