I tried something like this for my office chair. It was fine as a seat, but it raised my overall position on the chair vs the backrest and i ended up throwing out my back from the resulting offset. So, mind the chair you intend to use it on.

Just search for “coccyx pillow” and you’ll find the same thing from a thousand random brands, no need to buy some weird designer version.

And yeah they’re great if you need them for some reason or another.

“pressure relief”, huh? Makes it easier to fart?