That's why I said it's often up to the artist how they define such a thing. Technically, commercial use means you will gain some kind of monetary benefits from the used artwork, eg. publishing the book on Amazon, or you will be using it as a part of something widespread, like a marketing campaign with Google Ads or stuff like that. Most of the times, free publications like SH or RR are just fine since the thing is public. Naturally, you obviously should state the creator.
Always explain the exact use of the art if you are commissioning. And well, you using an artwork you paid for in your free novel or any other material also functions as an advertisement for the artist.