I'm a developer and I need to add a service charge at the checkout of my store, calculated as a percentage of the difference between the compare at price and price that the product is sold at. If there are multiple products in the cart, it should add each of these charges together into one service charge.
Since it's not possible to test out the checkout API to see if this is possible, could someone please let me know if there's a way to implement this using the checkout API, before I go through the process of getting an app approved by Shopify?
I would also like to mention that I've looked through the app store and not found any suitable apps for this. For example, the most popular app of this type is Order and Product Fees (https://apps.shopify.com/product-fees). However, this can only apply a basic fee, based on the actual product price. It can't perform the calculation that I require.
I'm open to any other suggestions that anyone could offer as of how to accomplish this.