The cost of using API is by your actual use of data.
The billing system is completely separate from a ChatGPT subscription.
If you’ve already put in a payment card method for API, but simply not used it enough in the past to get a bill, you have a post-paid account. You would not want the prepaid “buy credits” plan in its current state of bugs and inconvenience. Just generate >$1 usage causing billing from usage to put yourself in the “not casual trial moocher” category for future consideration.