There really isn't any rules except the ones you make up. Usually as the coordinator you would dictate how and when you want to be payed.
If the decision on what to buy is being made as a group I would suggest that you start by inquiring how much everyone wants to spend and how many boxes they wish to aquire. After that suggestions are made as to what to buy with the available funds.
If you have already picked out the box/boxes then really you just dictate as I stated earlier what the price is and what they are getting for that price and how they will be delivered.
It's pretty simple really.