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The problem with posts like this is that they give a very one-sided view of the situation and don't allow an uninformed reader (i.e., everyone other than the author and those close to them with direct knowledge of the situation) to understand the backstory and the reasoning for the pricing change.
I'm having to do Google searches to understand why this might have happened, and can only speculate. Is it that previously this company was eligible for a heavy discount as a nonprofit, and now something about that has changed? What has changed? We're not told anything.
According to their website, Slack offers discounts to charities [1] and educational institutions [2]. Does this organisation qualify now? Did they qualify previously? Has something changed in the organisation's status, or in Slack's policies, or has the organisation been misclassified and Slack has only just noticed? This post doesn't even attempt to explain any of those details.
I'm not saying that what Slack did was justifiable. It sounds like a terrible situation for this organization to be in, and I sympathize.
But without knowing any details at all about Slack's basis for making this change, this is the kind of post that generates a lot of heat but not much light.
[1] https://slack.com/intl/en-gb/help/articles/204368833-Apply-f...
[2] https://slack.com/intl/en-gb/help/articles/206646877-Apply-f...