Credits per conversion
When a job starts, credits for its estimated pages are reserved. When it finishes, you are charged for the pages actually converted and the rest is refunded. Every job has a receipt:
POST /api/v1/quotes. Quotes for uploaded files use the exact page count; quotes for URLs are a minimum, because the file is not downloaded to quote it.
Not enough credits
The job is refused with402 Payment Required, and nothing is reserved:
Limits
Over the rate limit you get
429 with a Retry-After header. Documents over the page limit are refused with 413 before anything is reserved; split them and convert the parts.