Fish are usually double or triple bagged and put in styrofoam boxes with packing peanuts in them. If it's during the winter months, they're usually shipped with a small heat pack. I don't know about the summer, since I would imagine a cold pack could go badly, but maybe with a small one. I've received a few fish via online shipping and they've all done fine including a little clown goby, so it seems to work. When weather conditions are too extreme, the companies usually notify you that your order will be processed when the weather gets better.