Here's how these work:
We'll pick someone to start the box (this is usually someone with a LOT of samples to contribute). They will pop everything they want to send in a flat rate box, make a list of what's in there, and send it to the next person on the list. The next person will take out what they want and do the same, crossing off what they take and adding their contributions. This will continue until the last person on the list gets the box, then they will send it back to the first person. The first person can use anything unwanted they receive for the next round of boxes.
Go here and join the group if you're interested in participating. It should be fun!
