Actually, shipping s'lings is easier than adults or juvies. The easiest method is to simply ship them out in the same small vials you used when you separated them out. These vials should have some substrate in them. Good sized vials for small s'lings are 3 to 12 dram vials. These can be purchased from Thorton Plastics. The good thing about 12 dram bottles is that you can keep the s'lings in there for quite a while. The 3 dram are great for shipping, but you can't keep s'lings in them as long since they out grow them faster. The larger 12 dram bottles may increase your shipping cost if you are shipping lots of vials.
When you get ready to ship, if the s'ling has not already made a burrow, use a pen cap or something similar to make a small burrow. Herd the s'ling into that and then place a moist (not soaking wet) tissue in the vial to fill it up. Take all your vials and securely pack them in a larger box. Then ship them out.