They don't seem to need to be sown immediately after they are ready (but definitely within six months helps), and they don't need to stay moist (unlike many seeds with short shelf-lives), so you can send them as any other seed, as far as I know. If you want to keep them moist, you can send it in a small zip-loc type bag with some moistened vermiculite or peat. I would recommend letting the people you trade with know that you should sow it during the summer, if they want germination by the winter (they germinate cold), or else they will have to wait until next winter. It seems that the drop in temperature to winter temperatures is their signal to germinate. Good luck!