It is too bad the installation instructions don’t come up on their site. That would be a clue on how it works.
Looking closely at the picture of the box, it appears there is only a red and black wire coming out of the HappiBox. This tells me it is a passive circuit (no active filter or op-amps) and having only two leads means it can only be one or a combination of a resistor, capacitor or inductor. My guess is it is an electrolytic capacitor used only to filter noise. The electrolytic capacitors are polarized with a plus and minus terminal, hence the red and black wires.