There are numerous Ask HNs about our demographics. Take a look at them. Compare with your domain expertise about people who run non-profits. The differences are fairly dramatic.
You'll never win an argument about pricing with someone who is not in the market for your product anyhow. Their opinions are not relevant. Getting $0 from HN readers will not compromise the success of your business, since there is no pricing strategy which will result in you getting more than $0 from HN readers.
I also think that 9.5% is less than 6% + 3.5% when considered from the perspective of the people whose opinions matter to you making your rent next month. For that matter, I think 10% would also also likely be perceived as less than 6% + 3.5%. In any event, you'll likely be additive on their existing revenue (or the customers will think you are), and 90% of free money is better than 100% of no money every day of the week.