I bought it at the time, somewhere in the 1990's. No internet and no cell phones at that time. Buying a box with floppies or cd was normal, even for an OS or basic tooling. Open source was out of reach, I didn't even know about the existence of the concept.

It came with a thick book, the reference manual for all C functions. That book alone was worth paying for.

Mix C was not that great, it miscompiled stuff on a regular base. I debugged a problem for a day before finding out the compiler sometimes flat out ignored basic constructions like i++

One day I found out about djgpp, and even it cost me as much as Mix C in phone costs, I never looked back. Quality was so much better. Still used that reference book a long time, though.