See if the folks down in the apartment office have an empty box about the size you need (take a tape measure with the dimensions you need). If they don't have one, check out the closest grocery store; they'll give you one for free, I'm sure!
I guess I can just send it in a manila envelope, taped to fit. That wouldn't cost too horribly much to send, I wouldn't think. And I have an extra...Jen gave me one to use for a present of Teresa's, but that ended up fitting in the box with everything else.