Yes this post box has a rear A4 letter slot with no flap to allow post to be deposited into the box through an existing letter plate on a door or solid gate.

The fixing kit that is included with this post box is more suited for a brick wall. You could install this to a plastic/PVC door using your own fixings.

Unfortunately the Forth Black post box does not provide fire protection.

The slot on this postbox needs to align with an existing aperture on the door. Providing this can be fitted flush to the door and in alignment it should be compatible.