You can do without the mouthpiece. A person brings his face closer to a special hole and exhales from a distance. Only the sensitivity of the device should then be high. Then, after each client, a quartz lamp is turned on to kill the bacteria on the receiving device.
This actually wouldnt works. Because concentration of exhalated air will depends on the distance between mouth and hole. The only way to exclude surrounding air is to breath inside straw. Vending machines will supply personal straw for each client. That is why everything will be personal and sterile.
This makes sense to use personal straws for each client. I wouldn't use it if everyone will use the same straw