If you exceed a free tier file conversion API service limits you will be offered to order a premium plan from this page: https://convertio.co/api/pricing/ .
You may choose any prepaid package that suit your needs.
These packages are not expired, you may order a big package and use it for years.
These packages are not automatically renewed.
These packages are one-time payment.
These packages are additive. You may order another package before previous runs out.
Note for OCR via API users: you may use OCR premium packages solely for OCR via API. Billing peculiarities of this feature can be found in the corresponding article.