Cost and Rate Limit of Sing in with Apple

Hello.
We are considering to use "Sign in with Apple" in our web service.
And we want to ask following three questions.

[Q.1]
How much does it cost to use "Sign in with Apple".
Sadly, we couldn't get any information from Apple's official web pages.

[Q.2]
Is there any Rate Limit to call "Sign in with Apple" API?

[Q.3]
When there is any Rate Limit, is there any way to increase Rate Limit ?

Thanks.