im in one and we're not allowed to have envelopes, but im not sure if its because we could purge into it or lick the sticky stuff off or what?
It's because "eating disorder" covers a wide variety of things. There are people in the centers who have Pica (they eat things that aren't edible). One of the most common Pica favorites is paper. I'd assume that would be the reason.
Maybe you could cut yourself with it or something?