What is the technology used for buttons in the US Post Office readers?
It is most likely some version of a tactile dome switch or membrane switch. Basically it's just a springy metal dome over some exposed PCB pads (although some are fully enclosed, stand-alone units - same tech, different package), when pressed, the dome caves in with a resounding "clack" and makes contact with the PCB traces, when released, the dome springs back, ready for the next press.

The same technology is used in TV remotes although they use rubber domes, hence the softer feel. Good quality tactile dome switches are surprisingly rare (which is probably why you haven't seen them anywhere else)