As far as i knew (or at least what my crazy post office tells me), the minimum charge for a letter going airmail is $1.80 for regular airmail, which would include a post card. if it wasnt in the envelope, i.e being sent as a postcard, could be less, something like $1.10. economy air should be less as well.......
i hate the postal system!
either way i hope you find a decent way to get it.