Quantity Increment Not Working

Hi, I have installed your plugin and it works perfectly apart from the increment, I have set to 10 but only goes up in 1’s?

https://www.pipedreamchristmascards.co.uk/beta/personalised/prancer/

We use a plug in called Fees and Discounts which set the unit price based upon quantity (which looks to be working correctly when adding up).

Can you help please?