Such as services offered by @Jerry Puckett or Clever Investor.That brings me to post direct mail.

We have negotiated with our vendors and have an invitation style, heavy linen paper, handwritten fonts, USPS permits, all down to approx 46 cents a unit INCLUDING POSTAGE.

Now, if I had basically the same MESSAGE on both letters and postcards, I might still get more calls from letters, but it won't be twice as many calls compared to postcards.And one last point, clever direct marketers mix up the media - maybe mixing letters and postcards over the course of a campaign, or (as we do) using different postcards - perhaps several in a series, or some with photos of houses, others with photos of people, etc.