The deed.
"Since they have to pay by weight, I decided to go with these itsy-bitsy teenie-tiny beads. Seriously, these things scatter like roaches when they fall; and they weigh more than glitter too, so it will cost them more to receive this package."
also, you're not mailing this back to the dept that sent it out-- that part is mostly automated, under orders from marketing and mgmt. you're actually mailing this to the dept of minimum-wage types who have to open all these up and deal with the contents.
suggestion: since we all get junk mail no matter what, just throw it in your shredder / recycling and stop taking it so personally.
AND anyone touching the envelope will KNOW there's something inside it, so you've thrown away the element of surprise you have with glitter.
Idea FAIL!