Among older direct mail consultants, using Courier or “typewriter” fonts is an unchallenged rule of design. But while I frequently use these faces, I also use others depending on the audience and product. I wouldn’t generally go with courier to a young business audience because it would scream “direct mail” and may interfere with the message. But for a hardcore direct mail responsive audience, a typewriter font will do nicely.

In design, there are no rules, only rules of thumb. It’s up to the designer to intelligently apply these guidelines to achieve the best result.

For the typical charity, I’d think a Courier or standard serif face would be a safe choice. Then again, I’d love to see test results on a highly legible san serif face such as Verdana.