Magento 2 – Mailerlite integration

After searching the web for a Magento 2 - Mailerlite integration I was unable to find one so I decided to build one myself.
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How to add Order in Multiple Quantities of X support to Magento 1.9

Magento built-in Qty Increments is a nice set of functionality. It forces user to buy specific product in defined Quantities of X. The problem is that it can only be set globally. How to change this?
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