Yes XYZ...when I asked customer service representative she was also confused, and she connected me finally to an Immigration Officer who suggested me this. I did not send using Overnight as well, because I was not very much confident about this (because it does not have PO box).
I sent my I-131 to (through Priority mail):
USBCIS California Service Center,PO Box 10485 (Attention I-131), Laguna Niguel, CA 92607-1048.
I wrote "I-131 application" on the top of the envelop also.
They received in 2 days but Notice date was after 12 days (Received date: October 1, Notice date: October 13). But I got the Notice.
Good luck