When i first received my red wigglers they came as follows:

-a cardboard box that had holes all around it that were made by poking with a pen/pencil.

-Inside he had placed a large ziplock bag that had tiny holes poked all over it (looks like the were made by a pin that hadnt been pushed all the way through (just the tip)
The worms were in the bag with enough bedding for them
to completely hide in and the bedding was from a running bin so it contained good food and moisture for them while in transit.

-I also had them shipped by bus (Greyhound) overnight.

They all arrived alive and well.

It is pretty hot out there right now. More like july than june. They don't need to be shipped in "dirt". Well soaked peat moss is good for shipping. You need something that breathes. A ziplock bag even with a few holes seems to me like not breathable enough. If the water can't evaporate then it can't cool itself. Old flour cloth sacks or something like that works best. Tightly woven but breathable. bag needs to be surrounded with newspapers shreaded for protection and insulation inside of a box with holes in it. Write on the box "contents- WORMS please help me keep them cool". By the way ship early in the week so they don't stay in a post office over the weekend.

I have seen a few different ways to package worms. They don't require plastic bags.

One company lines the cardboard box with several layers of newspaper, then places the appropriate amount of worms along with enough of their normal bedding to make them comfortable, then fold the newspaper over and close up the box.

You can easily get 2 lbs of worms plus bedding into the more cubic shaped flat rate priority mail box. I might be a bit worried about my worms shipping priority mail this time of year but I think that is how most growers do it.

The most important part is that they should not be over wet (as the box will start to fall apart) nor should they be too dry since that will kill them too. If they don't have enough of their normal bedding they probably won't be in very good condition if they arrive alive at all.

If you are worried about them escaping, you might just double box since the outter box will stay dryer and be less likely for the worms to work their way out.