A few days ago, I received RFE from USCIS. I mailed my response back to USCIS through overnight UPS. I sent it to the PO Box address required by the RFE letter. Actually someone from USCIS signed if off and there was an online update the same day.
My lawyer recommended mailing it to USCIS main address for the argument that it's faster and someone will sign it off. I don't think this is right sine the RFE letter states clearly that we should send the response back to them via envelope address.
Few takeaways from my personal experience:
1. you can send your response back to USCIS PO BOX address through signature required mail, someone will sign it off. This is the fastest way to get your response in front of them;
2. Follow instruction from the RFE letter that you received, what your lawyer says is not always right;
3. No need to put USCIS return envelope on the cover of your return mail to them as long as you put the envelope on the top of your response inside your mail.
Hope this helps