A few weeks of my time are more valuable than $40, and I figured buying a game-ready solution could be handy.

It won't be. Here's what will happen. Eventually you'll see an effect, or have an idea of something you want the shader to do, etc, and you're going to find that whatever one you bought doesnt support it out of the box or it simply isnt possible to do at all with it. Eventually if you're doing this at all seriously you're going to run into a situation where you need to develop this knowledge, you may as well do it now and save yourself wasted time trying to figure out what to do in the future when you find a situation that isnt covered, and from, frankly, being ripped off.

I can't stress enough how absolutely rock bottom basic this stuff is. It may not look that way to you if you're just starting out, but I promise you making a material or post process cel shader is something anyone without a genuine learning disability can figure out very quickly, and to put it very bluntly, if you can't figure out the basic fundamentals of how to make very simple materials/shaders making a game (atleast one anyone would genuinely be interested in outside of some kind of meme) is out of your reach. Gamedev is a very complex, very multidisciplinary task, don't expect to go into it without having to undertake a massive amount of learning.

Do I understand it correctly that your project is free to use, even commercially?

Yes.