Hey, I am developing an app to communicate between shopify and my 3rd party shipping and delivery app. I have a question Can I show the different shipping rates to the shopify checkout page through my app using APIs?
Have a look over our Carrier Service API here. There are some specific requirements around the process that an app needs to follow as well as store requirements, so I'd suggest taking the time to review the full doc.
That should give you a good place to start - Cheers!