Sort of implies that electricity was really the important innovation, to warrant the need for soldering? (Unless I'm totally missing the important application of soldering.)
I'd argue basic electric applications are pretty simple, and easily made by hand.
Sort of implies that copper smelting was really the important innovation, to warrant the need for electricity?
In all seriousness though, the printed circuit board might be a good one to bundle with that. The old methods of creating a mechanically robust compact electrical package were nowhere near as efficient. They're also very recent and very easy to make yourself, even if it's thin plywood with tinfoil glued on.
But I think that soldering (and the PCB, as @AWildC182 pointed out) facilitated certain widespread uses of electricity like radio and television that wouldn't have been democratized quite the way they were without it. i.e. TVs and radios would've had to hew to size and assembly labor restrictions of screw terminals and such.
I'd argue basic electric applications are pretty simple, and easily made by hand.