So we had two cases today where people had sent large reply envelopes to send back their unrequested partials using metered mail. This means that after they found out the weight and price of shipping the partial at the post office, they either had the post office stand the SASE before it went into the main envelope or they had one of those machines in their office that did it for them with red ink on the envelope based on the weight of the package.
The problem with this is, with metered mail you can't send it from a wildly different zip code than the one you metered it at. And because it was over 14 ounces, my boss took it to the post office to mail it, only to be told she couldn't, because it was metered in another zip code.
Long story short, if you're one of those people with a machine that weights and then applies a stamp via a meter and a red stamp thing, don't do it on your SASE. Find out the cost and put that much in stamps on the SASE so we can mail it back to you.
Some agencies don't make the trip to the post office or have an office person who does it for them, and just toss the returns envelopes that would require a trip to a post office because our country doesn't understand how actual postal security should work. So if you send an unrequested partial and it's heavy, don't expect it back, even if you send enough stamps to do so. Not everyone will send it.