I have developed a rough interface in my Shopify app. I want to know the process of interacting app and checkout page. After I set the state and fee for a particular product it should appear on the checkout page when the customer selects the state. for example, if a customer purchases a T-shirt from my store and his/her state is NY recycling fee should appear on the checkout page with the related fee and it should be added to the total. If someone knows please help me.