Ive sent a few gifts to friends in the states ( im in the uk ) and mostly theyve always made it through, the worst thing that could happen would be if customs ceased it and then they would only send you the envolope with a note letting you know some one tryed to send you whatever with the option to take it further (bad idea) ..
Best way to do it is make the letter as flat as possible and use a plain white envolope, aslo baggin up the stuff a few times to hide the smell is a good idea like lulu said but nothing will stop dogs smelling it if your unlucky enuff to get caught out.

peace.