GGtag is a reusable, reprogrammable ePaper-based badge with RFID.
Side note: It would be cool to allow exchange of payment information (such as lightning invoice) through sound rather than NFC.
Discussion
It sounds cool (no pun intended), but is there any practical advantage to using sound?
omg nuts
Easy two way communication, cheap interface, very programmable / hackable.
Can have good user experience, don't have to be so close to the terminal.
I think Google's payments (before Google Pay) worked this way in India.