I'm experimenting with an algorithm that uses MODULUS to ensure that the cart total is a multiple of a given number. This would be useful for retailers of wine, for example, where case-lots need to be achieved before checkout is allowed.
It works just fine all the while you have single bottles, allow mixed cases, and a pack-size of a given number... (we are currenly using 6 as the multiple on our test site, but you could change this to 12 in the admin panel.)
It's no where near ready, and I'm encountering some challenges!
But take a look at it at...
www.modshops.co.uk/wineshop
Add 5 bottles of anything to the cart... then add another bottle to make 6.... then take the total cart items to 11, then 12, then 18...