Thank you @jhilke-Dai.
It's about to be 30 days in next 4 days and i have not received nor mail receipt or return package. people whose document were submitted before and after my date have already received the receipt. I called uscis just to check if they can see anything under my name, and they were saying no, but to wait till 30 days for any further action. If the package is missing then that will be another torture to go thru.
Anybody who is waiting for the mail receipt and the wait time is near about 30 days like me?