Well US readers, it now costs 41 cents to mail a one-ounce first-class letter. In a small envelope. If you use a large envelope ("flats") -- bigger than 6.125" by 11.5" and no more than 0.25" thick -- that first ounce is now 80 cents. Bigger? Up to 12" by 15" and 0.75" thick and you're in package territory: $1.13.
I think the days of getting a promotional pen mailed to you are over. And, like Europeans, we may see more documents sent single-fold long ways. #14 envelopes would be helpful. (Some at Staples.)
As Chris Walton (Philocrites) pointed out, this is going to get expensive.
That said, these days, I would rather get almost all of my mail as email or as a PDF. (For periodicals, a link to download the PDF please.)
I'm looking for case studies here. Is there any church or church-related agency you know that relies heavily on PDF newsletters? I know the Linux/open-source world does it well.