Anything official gets noticed. Just last week I received a mailer from U.S. News and World Report. They used a window envelope with no business name listed on the outside envelope. Above the window a bold line of type read
POSTMASTER: PLEASE DELIVER TO ADDRESSEE.
This was sent at bulk rate, but they used a meter stamp so it looked pretty much the same as first class mail.
Now this piece got my attention, and being in the business I should have known better. Writing "POSTMASTER: etc." is wholly unnecessary becuse that's what the postman is supposed to do when you send a piece of mail. But this piece had me fooled. I thought it was important; it looked like the IRS or a long lost (rich) relative was trying to reach me. If it fooled me (and I should have known better) think of how it'll work on the rest of the public.
Another ploy on this "important envelope" trick is one stamped "OPEN FOR INSPECTION" and the envelope looks like it's been opened once and re-sealed. The post office regularly opens one piece out of every bulk mailing to make sure the contents conform to third class rules (this isn't a breach of privacy; they're supposed to do this). They then seal the envelope back up and put their "OPENED FOR INSPECTION" stamp on it. Guaranteed to be the first piece of mail the recipient looks at that day -- "Why is this piece so important that someone had to open it? Is something missing ... better check!" Everything in the envelope gets looked at. The receiver has no idea (unless they've been to a bulk mail acceptance center and seen this done themselves) that this envelope was really just opened at random.
A little twist on this at the mail creation stage would use a hand stamp saying "OPEN FOR INSPECTION" (notice I dropped the "ED" from OPENED) and then seal the envelope with tape, or seal it with sandy water to make the envelope look like it's been to hell and back. Your mailer will look like somebody tampered with it, the envelope will "appear" to say that it was opened, and your recipient will surely scrutinize your entire mailing piece.
Another offshoot on this is to rip open a side of the envelope (about 2/3 down to make sure nothing falls out and the envelope won't get jammed in the USPS machinery). Your addressee will think someone tampered with the mail and they'll check all the contents to make sure everything's okay.