I'm creating a website that only allows nonprofits and government organizations (parks, etc) to sign up and post service project listings. The accounts posting the listings must be verified to protect the safety of the volunteers who want to go do the service projects.
All nonprofits have an EIN that can be confirmed using an online database, but the downside is that anyone can look up the EIN and pose as the organization. In this question, the answer is to use financial transactions to verify the person, but with an organization, the volunteer coordinator (who would probably be making this account) likely doesn't have access to financial data (I've never worked with an organization like this so I could be wrong here).
I thought about email verification, but not all organizations have contact emails posted on their websites to confirm against, as well as the access problems already mentioned with financial data.
How can I verify that the person making the account is a representative of the nonprofit or government organization that they claim they represent?