If you're using one of these pre-paid envelopes to send a donation to a non-profit... don't put a stamp on it! What you want to happen probably isn't actually happening and that's no fault of the non-profit.
These BREs (Business Reply Envelopes) are an amazing tool for fundraisers to use, because all you (the donor) have to do is put in a check, seal it and drop it in the mail. The non-profit only has to pay postage if you drop it in the mail, which is great because your donor doesn't have to worry about finding a stamp and the non-profit doesn't have to spend a lot on unnecessary postage.
Once you drop it in the mail, the non-profit pays the postage AND a $.65 fee for each one returned (so $1.2 right now). I know this sounds like A LOT of money, but it's totally worth it, I promise (the easier you make it to donate, the more people will do it).
As someone donating to an organization you love, putting a stamp on the envelope (so you pay the postage) makes sense, but unfortunately, that non-profit STILL pays the $1.2, regardless if you put a stamp on or not! Once the shorting machines see the barcodes on the envelope, they stop looking for a stamp even if there is one there.
It is possible for an organization to save all those envelopes with stamps, fill out a form with the USPS for a refund, take the envelopes and the form to the post office and get a partial refund, but most organizations don't do that, so when you put a stamp on the letter, you're actually paying the postage twice and not saving the organization any money.
Why don't organizations do this? It doesn't make sense to for most, here's why.
So, say an organization gets 100 envelopes returned with stamps. Not only do they have to waste quite a bit of staff time sorting out those envelopes, filling out a dumb form AND going to the post office. But, to top it all off, after USPS takes their $35 minimum fee for the returns, the organization gets back a whopping... $20! AND they're still out the $65 in fees for processing that mail AND the staff time. Not worth it.
This might not be true for the mega-organizations who get thousands of envelopes returned with stamps on them, but that isn't most organizations.
So, long story short, putting a stamp on these envelopes isn't worth your time or money. The best way to help the non-profit would be to A) give online B) use the BRE and throw in an extra dollar C) use your own envelope + stamp.
P.S. And if you get junk mail with these envelopes, ALWAYS drop them in the mail empty! The people sending you junk have to pay the USPS, even if there's nothing in it.