Call USCIS and ask them for the address to mail back RFEs to the California Service Center.
Or maybe someone here who currently has an RFE from the California Service Center can tell you the address that's written on their self addressed envelope.
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Self filed AOS │Apps Received By USCIS - 3/18/19
│Biometrics Done - 4/11/19
Interview Scheduled - 4/24/19│Interview Date - 5/31/19│AOS Approval - 5/31/19
Permanent Resident Card Received - 6/8/19