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 and for URLs both use the page count read from the file, so a quote equals what the conversion will reserve. Quoting a URL downloads it once (up to 50 MB) to read that count, so it counts against the per-minute job/upload limit; a URL whose server does not report a size cannot be quoted, but can still be converted.
Convert what your credits cover
Send"allowPartial": true when creating a job. If your credits do not cover the whole document, the job converts as many pages as they do instead of failing, and reports where it stopped:
"startPage": 21 to convert the rest. You are charged only for the pages converted.
Not enough credits
WithoutallowPartial, or when the balance does not cover even one page, the job is refused with 402 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.