I did EB2 NIW and I faced the same issue, so we mailed it to them, NB: (NOT Email) normal mailing, my lawyer did it because am not in the USA.
The process is here.
https://travel.state.gov/content/travel/en/us-visas/immigrate/the-immigrant-visa-process/step-5-collect-financial-evidence-and-other-supporting-documents/step-7-collect-civil-documents.html