Hello, I only submitted documents online. I didn't see any instructions to send anything by mail during this step. I also found the below statement online. If I am wrong someone please correct me.
IMPORTANT: Do not send any documents to NVC by mail. However, you must bring every civil document you uploaded and submitted with your application to your visa interview. To submit documents, log into CEAC and go to the “Start Now” buttons located under Affidavit of Support Documents and Civil Documents.
https://travel.state.gov › immigrate
Submit Documents - Travel.gov