GP is in San Francisco. Standard notary fee is $20 in California, but some companies will subsidize it.
I feel sorry for those foreign countries where notaries are either rarified or unobtainably expensive, but it doesn't apply to this thread.
That said, I think Amazon's policy of notarization (at least in the US sense and fees) seems a reasonable policy from an account security standpoint. Not sure I can sympathize with GP who voluntarily added 2FA to their own Amazon account, then no longer had access to the phone number they previously used. $20 is not unreasonable for something that GP could have prevented with some forethought.
Outside the US this process can be lengthy and/or expensive as it can even result in official government stamps, translations, etc.