thank you everybody. i just called, and result is:

i called usps, and i was told that becaues it is a mailbox, so usps needs signature. i said i do not need a signature, but they said they need.

usps then said that the receiver (USCIS) comes to pick up the letter several times a day. so this appears to be good, if that is the case.

so now i keep my fingers crossed and hope that what the usps said is correct!

thank you very much and i will keep you update.