Have you tried setting up a free shipping code discount in Shopify? You can set it up to apply free shipping when the order exceeds a specific dollar amount. For example, if you want to let the customer receive free shipping on orders over $75NZD then under the minimum purchase order section you would select Minimum Purchase Amount and put in $75. If you want to allow free shipping on orders containing more than X items, you’d just enter that number in the Minimum Quantity of Items box. If this is something you’d like to use, just put an announcement bar notification to tell your customers what code to use.
I am not certain this will work with Printful, but it’s worth a try maybe?