The certification suite is open source software (produced by the OpenID Foundation mostly at their cost) and entirely free to use:

https://github.com/openid-certification/oidctest

Apple are absolutely 100% free to run the tool and fix any issues found without any payment to anyone except their own engineer's time.

(There is a fee to pay for the 'OpenID Certified' logo and to be listed as certified, but in my opinion just running the free tool and fixing any interoperability issues is a major benefit by itself.)