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?
Things might be very different in the US, but interchange is nowhere near 2.5% for a debit card in Australia. Maybe that's what is levied to the merchant (or more) but by the time it gets back to the issuer, after merchant service fees and on-charged scheme fees it's more like 0.5% for a debit card.
I could be mistaken but business models built on interchange (I think) have been the downfall of a number of 'neo-banks' from the last 5-10 years. More recently I've seen people adopting a commercial-classified card (or other) for these types of plays for the sole reason of it attracting a higher rate of interchange vs a typical consumer debit card.
I've been using https://superhuman.com/ for a while. It really is as fast as they say and with their focus on keyboard shortcuts, you can fly through your inbox(s).
It's not free but then again, that's a good thing I hope. Implies they plan to be around for a while!