When I logged into my USCIS account, the I-751 case was already added to my cases. Not sure if it's because I filled out the USCIS Online Account Number section in the I-751 form or not.
More updates on the timeline:
12/6: Package delivered at Tempe, AZ
12/11: Text message received "Your case was received with receipt #"
12/13: Check cashed
12/14: Receipt notice with extension available online