Solved

No engines for 16 tokens?

Unfortunately it's very unlikely you're wrong because there is no single option to buy engines for 16 token, but there is one for parts.

Yes, although it at least asks now if you're sure you want to buy X parts, or engines respectively. But it seems to be the nature of humans to just click away those confirmation boxes without bothering to read them, especially not carefully enough to notice there's something else there  than what you're expecting, helped by the fact that the font used is rather smaller, at least on this browser, than the ones used for most of the UI. Don't know how it's on the App and mobiles but I expect on those tiny screens this kind of error, both telling the icons apart and noticing the wrong word in the confirmation box, is made a lot easier there than here on a monitor.

What would be needed is a separate window for parts and engines instead of two tabs on the same with the less used parts one pre-selected.