How was the price computed? If Slack charging per user, how did this organization have so many users? Why is their new provider more favorable in pricing?
If Slack was previously offering a nonprofit discount, what happened to it? Did they decide that this organization was ineligible, or are they shutting it down in general?
Imagine your landlord increased the rent by 4000% and it's due in 5 days or you're out on the street.
Sure, they have the right to increase their prices, but there should be at least a month notice for something like this.
They spent multiple paragraphs complaining about Slack, and gave Mattermost a brief mention in a single sentence. I'd enjoy hearing praise about Mattermost if they're willing to provide it as well.