Two different things
A plan decides which capabilities your organisation has — white-labelling, the public API, advanced proctoring, the marketplace.
Credits are consumption. They pay for the work an interview actually does: generating questions, conducting it, transcribing it, evaluating it.
A capability you do not have on your plan cannot be bought with credits, and credits cannot be substituted by upgrading. They answer different questions.
Buying credits
From Credits, top up any amount. Purchased credits do not expire.
The balance
Shown in the header and on the Credits page, with the ledger behind it: what was purchased, what was spent, on what, and when. Every deduction names the feature that caused it, so a month's spend can be attributed rather than guessed at.
When you run out
Actions that need credits are refused with the amount required, before anything happens. Nothing is half-done and no interview is left in a broken state — a scheduling attempt that cannot be paid for does not create the interview.
Interviews already scheduled are already paid for and are unaffected.
Refunds
Credits are refunded automatically where the work was not done: a candidate who never starts and is closed as a no-show returns the stage's credits without anyone asking.