The results of a few stamp-related tests and observations:
If you’re using postage-paid business reply envelopes, test a “place stamp here” instead of paying postage. Clear cost benefit. Plus I’ve seen many programs also increase response and/or average gift when going from a nonpaid “courtesy reply envelope.” But if this works, test again in a few years. Organizations have seen results reverse over time. Go figure.
The only time I’ve heard about it being tested, removing the “your stamp saves us needed funding” line on a postage-paid business reply envelope INCREASE response.
Live stamps on your reply envelope will always lift response. (The late, great, beloved Max Hart proved and preached this many times.) What works best: multiple small denomination stamps. But the lift is not always enough to offset the additional cost.
Live stamps on outer envelopes will almost always beat a preprinted indicia.
I’ve seen a simulated metermark beat live stamps in some appeals. These were always cases where the organization was making the envelope look “official” … from the government … like tax groups or those dealing with government benefits.
The USPS has only one nonprofit standard-rate stamp, so it can identify your package as “junk mail” at first glance. You can mitigate this by overprinting a simulated cancellation. With this, try images that appear to be commemorative. Example: for the Red Cross, have a cancellation honoring Clara Barton.