Did you get your NVC case ID # and invoices number? On the website, there are the only 2 fees you need to pay online. Once you pay these 2 fees, you can print out the receipt and send it to the RFE.

"A copy of your DOS immigrant visa processing fee receipt (IV Fee Payment Receipt). The fee receipt must show that the fee has been paid in full. Documents such as the Immigrant Visa Application Processing Fee Bill Invoice, Affidavit of Support (AOS) Fee receipt or a receipt showing the payment is in process are not sufficient and could cause delays in processing your case. See the National Visa Center website at nvc.state.gov to learn how to obtain your immigrant visa fee receipt."

The quote is from USCIS filling tip for i601a