Joanna, I'm further south than you are, but I'm assuming they sent you seeds of the common milkweed(Asclepias syrica)?
Here's a link to photos of the sprouts.
https://duckduckgo.com/?q=comm...
Many plants look alike in the very beginning b/c they have only the first two little leaves called cotyledons. The plastic jugs help retain some moisture while at the same time keeping the little plants a bit warmer than the surrounding air.