So you would like to get notified as soon as there is activity in your mailbox? The SinMan Mailbox Chime & Alert System has you covered. It has a range of 150 feet and sends you instant notification when you have mail. It can be installed on mailboxes of all sizes and shapes.
The mailFi sensor has a range of 150 feet but you can extend that with a repeater. More info is available here.