Force minimum amount of a mixture of products

Hey together!
I just wanted to use Zen-Cart to build a webshop for a vine saler. But since he has special order conditions i ran into a problem:
It should be possible to choose any bottle of wine out of the product categories "red wine","white wine", but with the conditions only letting the customer order multiples of six of the total amount of bottles. This restriction is for logistical reasons, since the bottles are shipped in standardized boxes for exactly 6 (6,12,18...;-)) bottles. If it isn't filled completely the insurance company will charge more for the transport.

Is there already any contributions (which at least i haven't found yet) for this special problem?

Cheers from Berlin,
Thomas