Is this a K-1 Case? I ask because your timeline is blank and I don't know what case you filed. If it is a K-1 case, you do not have an invoice number. They are not issued to nonimmigrant visa applicants. You will just mail the documents back in according to the instructions. The "invoice number" part of the notice is in there because they give the same paper to immigrant and nonimmigrant (K-1) visa applicants that get 221g refusals. If you want you can put a note in with your mailed in reply saying something like, "Here is the 221g response. We were unable to upload scans of the documents to the CEAC portal because, as a K-1 visa applicant, we were not issued an invoice number."