Gaze Security via Amazon offers the Enklov Wireless Doorbell for $17.99. Coupon code "R8X9WKXL" cuts that to $9.89. Plus, Prime members bag free shipping. That's $8 off list and the lowest price we could find. It features 30 chime melodies, LED alert, and 330-foot operating range.

Update: The price has dropped to $13.99 before coupon, $7.69 after.