Checkout in Multiples of 12

Hi, 

I am working on a Squarespace store for a winery and need add some features to the checkout page. 
The customer's order cannot be processed unless it is a multiple of 12. 
I've been checking out forums for documentation of similar things but I haven't found much involving Squarespace pages. 
This website has a version of what I need (https://sage-hills-wine.myshopify.com/cart), however, I looked at their code and noticed that the "Order Limits" feature is integrated with Shopify, which I am not using. 

Does anyone have experience with this involving merely some custom code inserted via Code Injection, or should I be looking into alternative e-commerce software for this level of customization? 

Thank you   

I would look at alternative e-commerce solutions. Whilst it is possible to code a solution you cannot add any code to the checkout page on Squarespace. This means that customers would still be able to change the quantities in their cart on the checkout page. 

In my opinion Shopify is probably the best solution. 

@ThompsonWebDesign
Thank you for this response! 

Just to clarify one thing: 
When you say Whilst it is possible to code a solution you cannot add or change the checkout page in Squarespace, do you mean In principle, one could code a solution, yet Squarespace does not permit you to add custom code/alter code for the checkout page?
As in, you cannot customize the checkout page anymore than the available customizations in the "Commerce" section of the configuration panel?   

Thank you 

@Crisp to clarify, you could add code that prevents a customer reaching the checkout until they have 12 items in their cart. The problem is with the next step - the checkout page. On this page there are buttons to allow the customer to remove items from their cart. You cannot add code to the checkout, so you cannot override this behaviour. 

@paul2009
That makes a lot of sense. I appreciate your help!