Calling Gradio Spaces via the API is free and best effort. Only for Zero GPU Spaces, there is a benefit from a token with a Pro subscription. (There is a version-dependent bug.)
It is recommended that people who want stable operation use Endpoint API (dedicated) etc.
The fee is paid by the person hosting the Spaces.
If you’re worried, ask the following support.
Payment related: billing@huggingface.co
General: website@huggingface.co