Might I make a suggestion...
Talk to MCE and ask about having a fixed time frame for this group by like you've proposed, but rather than have people give you their CC info and you placing one large order just let them call their order in to MCE. MCE could easily hold all group buy orders until it expires so they can have a definite total to determine pricing and then process the orders accordingly.
Consider me in, but I'm not trusting my CC info to anyone. (no offense)