I've mailed other macroalgaes before. Basically, put some in a fish bag with a little water, seal the bag with a rubber band. Put the bag in another bag (in case of leakage), then into a box with some packing (styrofoam peanuts or crumpled newspaper). Send by Priority Mail (three-day service). I generally keep boxes from previous things I've received. I believe you can get Priority boxes at the post office.