You can send glue, just make sure it's CA (Super) glue. I have sent acrylic paints and, to date, have not had anything returned to me, and as far as I know everything arrived intact. I think the key is being vague on the manifest, hence the "model kits and supplies" instead of "model kits, glue, paints," which might provoke all kinds of unnecessary responses.
One thing I try to do, since I'm never entirely sure where the boxes will end up, is send enough supplies with each group of kits so that they could be built with just what's in that box. Every box has some kits, glue, paints, brushes, sanding sticks, etc.