Ill try to minimize the spoilers to a minimum...

So, I eventually finished the Dark Brotherhood story, I've got the new sanctuary and I went to Delvin to buy stuff. At that point the Letter of Credit would have proven very useful. So I went back to the old Sanctuary hoping to find it in some cabinet. Not a chance. Did it have any use at all? If so did I have to pick pockets Astrid for it? I mean.. It was 15,000 gold worth in services… Not that I'd need them really, I reached the 100,000 gold limit already and i find the commerce thing just useless in Skyrim, but still it was better to have them than not. Is it gone forever?

Yeah I think so. I just assumed Astrid spend it on a few things already without telling anybody about it. Or perhaps she bought a single painting and somebody shot a firebolt at it....