Printers; printers for use with computers; printers for use with point-of-sale terminals; thermal printers; mobile printers; receipt printers; cash drawers for point-of-sale systems; cash drawers for point-of-sale systems for use with computer printers; cash drawers for point-of-sale systems as computer peripherals; cash drawers for point-of-sale systems with built-in printers; point of sale computer hardware and peripherals; magnetic coded card readers; smart card readers; magnetic coded card writers; smart card writers; magnetic coded card reader and writers; smart card reader and writers; near field communication (NFC) reader and coded card writers; radio frequency identification (RFID) reader and coded card writers; computer programs for configuration and control of computer printers; computer programs for configuration and control of cash drawers; computer programs for configuration and control of point of sale computer hardware and peripherals; network interface hardware for controlling cash drawers in point-of-sale systems; a computer hardware device to be connected to an interface of an electronic device for emitting radio waves and computer software for measuring radio field intensity of radio waves emitted from the computer hardware device; computer software for controlling printers and mobile devices for users to obtain electronic receipts from retailers