I think the concept of paper notebooks has been suggested once or twice, but I don't remember the reception to that. However, for the idea of being able to add to a letter... well, it's what it sounds like. Have you ever passed notes in class using the same piece of paper? I haven't, but that's the same concept I'm suggesting here: a way to ADD TO a letter, but probably not erase. Maybe there could be some messages when you look at the letter/page, like 'there's a lot of space left to write' 'more than half the page is written on' 'there's space for a few more lines'.
I have no idea how difficult or easy this would be, whether there are reasons why cheap, reusable writing methods haven't been introduced, but I do think that this would be useful for more than just me. And heck, maybe we can add writing on flyers– recycling, right?