Be polite, say goodbye!
So how did he do it? He used a thing we all hate about the web, the “Subscribe for 10% off your next purchase!” popup. Or as we call it in the industry, an exit intent popup. But he used it with no other purpose than to say goodbye, and that’s really nice!
I’ve implemented a version of it on my site as well, you can see how it works by moving your mouse outside of the browser window. If you are reading this on a phone, here is a GIF showing it in action.