Right - there is so much context missing. Extrapolating just a little, it looks like the story is something more like "We had too many users for our existing free plan, so we switched to a paid plan that had a first-year discount of 97%. We didn't really pay attention to the fine print, and now we've got a nasty letter demanding lots of money."
Not defending Slack / Salesforce. You just can't deal with them with that level of naivety.