Legal Question in Construction Law in California

I'd think 'yes' because first of all, the post office form says signature by addressee or agent; and second, anyone who pro-actively accepts deliveries on behalf of another is an ostensible agent for that purpose.

Most likely it is fine, but it was a mistake to send it by a method requiring a signature. Just send your 20-day notices by certified mail. You don't need the return receipt.