Either the certificate is the same every time and therefore it's an identifier.

Or the certificate isn't the same every time and therefore you can generate a whole bunch of them and give them out for $2 apiece.

Or the certificate isn't the same every time and also isn't anonymous so they can trace who's doing that.

You don't have to reuse the same certificate for several requests. You can get a new one for every request, for every person who is asked to verify their age and pays you $2, and if they're actually anonymous, there's no way to know you did this. Is a rate limit part of the proposal? Can I only sign up to one adult service per week?

Unless you meant the requester's real identity, in which case... we're back to not anonymous.