The extension is a one time purchase and it runs locally, all of the tools for that can be found in the Sprite menu, or in Help, Retro Diffusion Scripts.
It sounds like you may have been confused and used the "Retrodiffusion.ai" tab, this section lets you also use the website, which requires credits. This is why they are labeled "API Text to Image", because they are using the website API. But it is only an option, not something you need to generate with.
You can use all other tools in the extension without signing into the site, buying credits, or even having an internet connection.