Wrap them in a DAMP--never soaking--paper towel and place them in a zip bloc bag. If the bag isn't the double-lock type, it's a good idea to tape the opening of the bag (or better yet, invest in a $30 ebay plastic sealer and some basic poly bags to ensure that the bags won't open during transit). Make sure to remove most of the air, but not so much that the plants are being crushed. Fill the box with shredded newspaper to protect them and add a bit of insulation (styrofoam box lining should be used in colder climates). It's also a good idea to label the bags with the scientific names of the plants that are inside so the buyer knows exactly what is what. In my experience, both submerged and emersed-grown plants should be shipped the same way, but perhaps a bit less moisture for emersed-grown specimens.
A lot of people make the mistake of shipping plants in too much water (or in a full bag of water, which you should never do!), and usually the plants end up in terrible condition or DOA. Always ship priority (or first class if the package isn't going far), and always use a box. In most cases a padded envelope isn't a good idea, though i've seen some people use them with success. Finally, as i said earlier, make sure to not pack them with too much water-- that is perhaps the most important thing. Other than that, there isn't much of a science to it.