How it works is very simple: attach the sensor to the inside of your mailbox with the included double-sided tape, and plug the receiver into an electrical outlet wherever in your house you are most likely to hear it. It gives you an audio signal (four beeps) as well as a visual signal (a bright red LED) when your mail is delivered. It has a range of up to 450 feet, and you can adjust the volume of the alert. The light stays lit until you hit the reset button, so even if you're not home when it is delivered you will know when you get in the house. It works on curbside, wall mounted, and slot type mailboxes, but is not for use on mailboxes with vertical hinges. It includes a 12V A23 battery, so it's ready to go just as soon as you set it up.
Not only will this save you walking out to the mailbox unnecessarily, it's also a handy way to monitor if the neighbourhood kids are wreaking havoc with your mail. You can get one today for right around $45.00 US.
Source: Gizmos For Geeks