If you are developing with the Shiptheory API, you will need to create a regular Shiptheory account. Once registered, select the Pluto plan, which is free and never expires. This gives you access to our API.
Additionally, if you do not have a Carrier account yet but wish to test creating labels and tracking numbers, you can enable Shiptheory's Address Only Labels. From the settings page, set Generate Faux Tracking Numbers to Yes to have the API generate and return fake tracking numbers. For more information on this feature, see Address Only Labels.