I'm not too familiar with Slack pricing but it suggests in the Fair Billing policy[0] that they bill per active member. Without any discounts, the Pro pricing is $7.25 per active user per month, if paid annually.[1] If they are needing to pay $200,000 annually, then I think that means they have over 2,000 active members in their Slack which does not sound like a "small nonprofit" to me.
[0]: https://slack.com/help/articles/218915077-Slacks-Fair-Billin...