On 20/04/2015 16:54, MM wrote:

No, the issue is that I need to catch the postie when he's about to
leave a card because a book or similar package from Amazon is too
large to fit through the flap. I've got TWO bell pushes, but sometimes
the dozy geezer doesn't ring either, he just taps on the door and I
don't hear it. But if he's maybe already stuffed a letter through,
then I would already have that alert to go on, rush downstairs and
open the door as he's departing whereupon he's pretty much obliged to
come back and hand me the parcel. And if he's only stuffing the
collection card through, then that would be enough to trigger the
letterbox flap alert.

You need a PIR connected to a chime, then it detects him walking up and
not pushing anything through the flap.

You can buy them as cordless battery operated.