Fairly sure Xero use NetSuite (or did- maybe they've since moved) and apparently went to great lengths at the time to make the invoice look like it was generated from Xero.
We looked at this exact setup recently but realised you can't suppress the unsubscribe link from any email customer.io sends.
You can set a flag so the user can't unsubscribe if you have a legitimate reason (e.g. transactional email) but I'm not a fan of the customer experience here as unsubscribe should mean that, unsubscribe.
One thought I have on your model is make your entry price some sort of bundle. Your product is designed for workflows and you can't really have a workflow with 1 user so already you are at 2 users to start.
I am sure you can play around with this but it feels like this product would benefit from getting into as many users hands as possible when you are establishing yourself in a client business. Atlassian is an example of this where all of their products (or almost all of them) are $10/10 users and once you hit 11 users, it goes up quite a bit. That might seem like quite a jump but imagine what you are paying in salaries at 11 people vs. what your JIRA subscription now costs?
But to clarify, they gave a service to people/teams who maybe they shouldn't have. Did the people signing up/using Slack know that? Maybe. Who's responsible to ensure it's enforced?
This is not about blocking accounts of people in Iran, which is a totally legit thing to do. This is about blocking someone who is of Iranian origin, but claims to live in Canada for years and has no business connections with Iran whatsoever. Probably it's some automatic filtering hitting a false positive, but question is how that works, what criteria they use to search for "suspicious" accounts? Have they been given the list of accounts by some US agency or they track and profile their users? There's a lot of confidential business info floating through Slack channels, so it's a huge deal for all of us if they data-mine on that in some way.
At least they aren't hiding that anymore.