There's currently only two companies that will knowingly ship beer: UPS and Fedex. Both have similar rules and regs for shipping liquids (check their websites) but beyond that, you can send them out however you wish. Most people ship them Ground, as cheaply as they can because bottle trade boxes are AT LEAST medium sized boxes, if not large ones, and heavy. There should be a shipping FAQ, but if not, check the shippers websites for instructions on how to ship perishable goods and liquids (or ask your trade partner). I would suggest taping the caps/lids down, wrapping individual bottles in bubble wrap, heavy uses of packing peanuts and newspaper to keep bottles from clinking and an inner leak proof liner.