Hi,
I know there is a free Tier, and when a user runs out of credits he/she may then purchase a paid plan.
However, is much more efficient and persuasive in terms of marketing, to offer the users a limited free trial (for 3, 5 or 7 days, or for a certain amount of credits), and after it expires, they automatically enter the paid tier.
Of course, that would mean that in order to access the free trial, they have to input their credit card details.
This sales process is what most SaaS or apps are doing nowadays, as reduces friction for customers.
I hope you’ll consider it, thanks!