here in Canada we are blessed with ideas and some companies would actually go far enough to make something out of ideas.
This one is called "draft dodger" mail slot, and Rona has it.
Mail Slot | RONA
http://dhdtvspecialtyhardwarestore.ca/welcome.html
I bought one and installed @ my parent's door (which, the original metal flap thingy just let drafts through like crazy)...and it worked beautifully.
Yeah, a bit costly but works (2 flaps: one on the outside; one on the inside. Must lift the outside alu flap in order to slip mail into the inside).
Quest