They do need to be kept fairly warm, so again you'd have to wait until your night time temperatures are above 55deg. or so. If the plant sat in your mailbox overnight on a cold night (and where you are, nights can get cold even in summer, right?) they would not be happy.
Once they're grown they may be a little more hardy but you'll still need to bring them in for the winter once the temps are cold like into the 40's at night.
