Have a token- or ticket-based approach:
1. If you don't have a token/ticket (or need to buy more), there's one line for that.
2. Folks with tokens hand 'em over in exchange for bags, preferably right at the truck so no fetching (you effectively crowdsource that bit).
Money is separate from the actual moving of product then (have to do the whole "no refunds/no cash value" thing on the tokens). For people that pre-buy tokens, the line will be lightning fast. For everyone else, it'll still be faster than before and you can flow people to the registers or to pickup as needed to deal with demand.