Abstract: A payment instrument unit that can be connected to a vending machine, comprising a connecting member for connecting the payment instrument unit to the corresponding member of the vending machine, and a means of communicating with the vending machine by using a certain vending machine protocol, is characterized in comprising a first means that comprises a first data transmission device for receiving a contact request from wireless terminal equipment, and a second means comprising a second data transmission device for authenticating the user of the said wireless terminal equipment.