Hi bb
Good to hear that your case is all good at this point and waiting for IL. Since you mentioned the letter sending/delivering stuff. Just FYI, my spouse had received bio letter,485 receipt, IL letter all smoothly. But after interview, he got RFE(we don't know this when we finished interviewed that day). We wait for 2 weeks and nothing happened, so we infopass and ask infopass officier whats wrong. Answer is he got RFE and the RFE send out 2 days after interview. And return to FO because it cannot deliver (which is nonsense because we got all the letters before with the same address), then FO send again, then got return back to FO again with the same reason..... So the RFE send twice and returned twice back to FO!
But thank goddess we infopass and the officer print out the RFE to us right away. But he cannot explain why all previous mails can deliverable and the RFE cannot.... Just sharing my experience, hope it helps. Good luck!