Amazon offers Prime members the
Honeywell P4 Premium Portable Wireless Door Chime with Push Button for
$19.99 with
free shipping. That's the lowest price we could find by $18. Features include six chime tunes with adjustable volume, six visual alert icons, a self-learning code system, and 450-foot operating range. It comes with one push button, but it can connect with up to six Honeywell push buttons, motion detectors, or door contacts simultaneously. (Additional transmitters sold separately.) Four C batteries are required, not included.
Note: While the product title notes that the item comes with three push buttons, multiple Amazon reviews attest that it only comes with one. This item may take an extra one to two days to ship.