Where can I study an example or instructions for integrating an API into OpenCart?

Question

API

There is a delivery api https://api.shipper.io/docs , I don’t quite understand where to integrate it.

Answer 1

You need a module, a kind of intermediary that would request data from the delivery service through this very API, and only then would make the appropriate amendments to the store database.
Simply, if I'm not mistaken, before 2.x, Opencart had no built-in tools for interacting with third-party services at all, everything is done through modules.