Hi Drivet,
CarrierService is indeed one of the resources you might need to use for enabling your client to integrate custom shipping services into their store. While the documentation might not mention access keys explicitly, these are usually provided when you spin up your app and are used to authenticate your API requests.
To get started, you can use the Shopify CLI to generate a boilerplate app, and then request the scopes you need, install the app on a development store, and start making API calls to create carrier services.
Hope this helps!