I have been involved in the personal computer world since the late 70s, reading about S100 bus computers and CP/M, but I don't remember these "checksum" tools for entering programs, but I was much more in the TRS-80 world, not Commodore or Apple, maybe that's why? Anyway, what I do remember is one magazine (Kilobaud?) that put flexible 45 RPM vinyl records inside the magazine, and you would "play" the record on your turntable, connected to your cassette interface.
I also remember in the TRS-80 world there was a tradition of making "one line" programs - they got to be quite sophisticated but were really just "scrolling games" in most cases.
I remember typing a massive fruit fly genetics program into an Apple ][ one winter break as extra credit for my biology class - it was in Pascal and was an ungodly amount of typing...