Some of the products need to be shipped in box of six or twelve bottles (as appropriate).
These box are called Winebox , and they must be complete to ensure the integrity of your products during shipment.
It is not necessary that you buy six or twelve of the same products, but it is necessary that you complete the Winebox, even with different products.
In the product page you will find the "Complete winebox" button which will allow you to automatically fill the box with enough bottles to allow you to ship them.