ChatGPT Plus is a $20 monthly subscription to an upgraded ChatGPT only.
API is billed by usage by the cost of the model.
This is where you see and fund API credits to pay for services:
https://platform.openai.com/settings/organization/billing/overview
This is a pricing overview:
https://openai.com/api/pricing/
If ChatGPT Plus does not appeal to you, you can request a refund and cancellation using “help” in the API account site in the first days. Cancellation you perform in ChatGPT will still leave the paid month of subscription.