[TYPO3-commerce] Validation before checkout!

[Idar Creative]

Hi List,

We have the following problem/challenge which i want to discuss with you!

We are developing a commerce webshop for a client that sells exclusive 
wines.
A mandatory is, that it's only possible to checkout the basket by 6 piece 
(amount)
So you can order 6, 12, 18, 24, ... bottles
Example: you can order 6 bottles of one brand or 1 bottles of different 
brands or in between!

Before  (or after) clicking the "to checkout" button, I want to validate the 
amount.

How can we resolve this matter?
Any suggestion will be great, because I don't have any clue rigtht now.

Kind regards,

Idar