I would just assume that the vendor's payment app would just play a chime and maybe a readout of the amount received. Every payment app I've used has a notification with name and total when I get money. I'm pretty sure Android can already handle this anyway. Connect the phone to a speaker if you need it louder. Seems like a much cheaper solution than buying a speaker with a built-in modem that you have to pay a monthly fee for unless you just don't own a smartphone.