If you do not need enterprise use cases (single sign-on, higher level SLAs, shared authentication, etc) and your scale fits within our defined constraints, then free is the price.
ESPECIALLY since this would become a key part of the infrastructure / how my app gets its stuff done. Replacing your service with a competitor or something homebrew would be a major pain, especially when I find myself unexpectedly popular.
there is NO WAY I would ever use or recommend a service that refuses to tell me what my potential long term costs are up front.
It's too much of a gamble with a business or worse, a hobby project that isn't making me any money to compensate for this unexpected "balloon payment".
(I am not affiliated with this vendor, I just deal with software vendors a fair bit.)