Appreciate your help today. BTW, I'm leaning towards uploading this RFE response. The taxes and proof assets forms would probably be something like 500 pages minimum to mail. (Complicated taxes). Also, I was able to upload passport style photo for the I-130 petition that was submitted online and they didn't have a problem with that (so there is precedent for them accepting the passport photos online).
Another factor is influencing my thinking on uploading vs sending via snail mail. On my wife's USCIS account, there is a button we can click to upload the RFE response to this specific RFE. Since both the I-864 and the I-485 are both submitted by regular mail orginally, if they didn't want us to respond online to an RFE for those specific forms, I don't think they'd have a button ready made for us to click and upload docs.
But I will ponder that decision a bit more.
Really apppreciate all your perspective. Crazy thing is how you get different answers for same quesitons on all of this.