Yes. In my opinion, and very few will agree with me, ONLY if you FILED your application ONLINE are you ELIGIBLE to reply to RFEs ONLINE. While you may be ABLE TO doesn't mean you are ELIGIBLE to so you should not reply electronically unless you filed electronically.
They did not send you an RFE for no reason. Don't be like the many others and be so sure of yourself. There is at least one error and statistically, likely many more than one. Time to do some research through the I864 RFE threads as advised above. You'll need to examine your forms and documents very carefully asking yourself what am I trying to prove with each piece of evidence and ensure the numerical amounts match your form.
#1 problem: Self employed sponsor's and not knowing how to prove income. I'd advise you not to use a SE'd sponsor.