Plussino via Amazon offers Prime members the Plusinno Physen Series Wireless Doorbell for $18.99. Coupon code "6HHD3CL8" drops it to $11.99. Plus, Prime members bag free shipping. (Non-members can spend $35 to bag free shipping; otherwise, it adds $6.60.) That's $21 off list and the lowest total price we could find. It features a 500-foot range, 52 melodies, and adjustable volume.