Pay as you go: buy credits when you need them, use them within 365 days.
Buy another pack whenever you run low and the credits are added to your balance. For more than the biggest pack, buy several.
Pay-as-you-go credits are valid for one year from the day you buy them, so a busy month and a quiet month cost the same. They also open the paid models, and with them the API.
Packs work alongside Pro or Max: the credits of the plan are spent first, then the credits of your packs.
One-time purchase
1 year (365 days)
Packs stack
The web