Our price add-ons use draft orders which aren’t compatible with the discount code box on the checkout page. Shopify has known about this issue for years but hasn’t fixed the problem.
We worked with KartDiscount to make our apps compatible and allowing you to add discounts on the cart page.
You can check out a working version of this app on our demo site.
Click here to install KartDiscount and add a discount code box to your cart.