Did you have a special deal with Slack? I don't understand how they can just increase the price with a few days notice?
If they determined that Hacker Club violated some terms of the nonprofit demanding they move to regular or be kicked out seems not as bad
Although frankly this is a good lesson for a bunch of young hackers to learn.
If slack found out that the company isn't really a non-profit, or that it violated the requirements in the non-profit agreement (such as promoting discrimination) it would justify a demand for immediate payment in my opinion.
Not defending Slack / Salesforce. You just can't deal with them with that level of naivety.