If the response-to-advertising mail were also added to advertising mail it would have resulted in double counting.
If the response-to-advertising mail were also added to advertising mail the items would be counted twice but the carriers don't mind.
mnli hypothesis: If the response-to-advertising mail were also added to advertising mail the items would be counted twice but the carriers don't mind. premise: If the response-to-advertising mail were also added to advertising mail it would have resulted in double counting.