Less so with non-profits, more for things like catalogs, free magazine things, etc.
Like can you power a "wood" stove with enough dense paper? And if you're getting 100 heavy catalogs a day, will the USPS keep delivering them?
Or is there some kind of practical limit that the USPS won't do residential delivery of commercial mail beyond a certain daily amount?
Surely there's some reason you can't heat your house this way.
Loose paper burns quickly so you'd be filling up the stove constantly.
[0]https://www.amazon.com/Pine-Mountain-Traditional-Campfire-Fi...
The sender is paying, and they are generally pleased people are giving then money.
If you weren't inclined to send in another donation, you could remove those stamps and use them for your own mail!