Packed by the case means you must accept whole cases regardless of how much of a particular product or flavor you sell. Consider our bulk cookie dough tub fundraiser as an example. All flavors come in six tubs to the case. Breaking down cases is not possible, so you cannot mix flavors.
Suppose a group sells 33 tubs of chocolate chip. Because all cases come in cases of 6 tubs, we must send 36 tubs (6 tubs/case x 6 cases). So, the group would receive three extra tubs.
The group would be responsible for paying for the extra three tubs. Once they sell them, they keep the additional profit. Or, they can use the extra tubs to fill late orders or use them as incentives, or volunteers can resell them and keep a portion of the profits.