One cost that people might not think of, because it's not a direct monetary outlay at the time of mailing, is the extent to which the efficiency of the marketing dross (presumably?) you're sending is lessened because of the annoyance of recieving two identical copies of it.
Hard as Hell to quantify, of course... But I know that shit like that, being a sign of inefficiency and thus stupidity (not to mention being a waste of natural resources), pisses *me* off quite a bit; and since I'm hardly unique in this respect, there's *some* opportunity cost incurred there.