Adding 'Additional Item Fee' For Shipping ?

So I'm using the print fulfillment site Printful.com to sell mugs on website but I've run into a problem when a customer buys the same product but 2 different designs. 

Say I have 10 different designed mugs up for sale on my site, and a U.S. customer orders 2 different mugs, shipping is $7 and every additional mug is $4 so the total shipping cost for 2 different mugs is $11. But on my site, when a customer orders 2 different mugs the shipping is $14.
Does anyone know of a work around other creating 1 product page and listing all 10 mug designs as variants? Because that isn’t going to work for me especially since I plan on having over 30 unique mugs in the near future and I want them all to be separate from each other, so it makes it easy for the customer to see each mug on their own. 

Is Squarespace working on a shipping feature like this?

Flate Rate shipping in Squarespace allows a "Base" and "Per Item" while Printful allows "First Item" and "Additional Item". You can solve part of this with math.

For instance, Printful t-shirts cost $3.99 for the first item and $1.50 per additional item. That translates to Squarespace as $2.49 base plus $1.50 per item.

Unfortunately, that only works if you only sell one type of item. Even with "Advanced" Shipping, Sqaurespace doesn't allow different shipping rates to be applied to specific items like Shopify and any other mature ecommerce platform does.