Then they should have chosen a chat server that has that as the business model.
The decision maker didn't. They chose a product that did not offer that option, then negotiated the $200k down to $5k.
Slack was obviously unsuitable for them because Slack does not offer what they wanted (free for non-employees), but the decision maker blundered on. And now they want sympathy.
> The decision maker didn't. They chose a product that did not offer that option, then negotiated the $200k down to $5k.
And in doing so Slack added that business model. And seemed happy about it.
> Slack does not offer what they wanted
They offered it to them.
> And now they want sympathy.
They deserve plenty of sympathy for Slack not giving them any reasonable warning as they torpedoed the deal. And it's not like they were draining Slack's resources or doing anything that made this an emergency.
It doesn't matter that an alternate method of counting would be a lot more. They paid a reasonable amount for what they got.
$200k for this service is a joke, not the 'real' price.