I would offer them a discount and think about donating to them once a year a portion or all of their subscription if you like the cause. Free accounts could be

I would offer them a discount and think about donating to them once a year a portion or all of their subscription if you like the cause. Free accounts could become a burden over time and could lead to people just trying to sign up for a free ride. Add a link to your pricing page that says Are you a 501(c)3 non-profit contact us for discounted pricing. See how much interest you get. You can decide on the discount when the first email rolls in. (Setup a new contact page/form that indicates to you they are interested in non-profit pricing). Make a difference.