Thank you @Cathy_Sarisky here is the downgrade guide: https://help.mailgun.com/hc/en-us/articles/9790737438747-How-Do-I-Downgrade-My-Account-
I want to update this to make it a little more clear as to what I did.
When creating an account at Mailgun, you will be required to upgrade to a trial, which lasts for 30 days. You will need to link your payment card to proceed.
It is only after this, and a few days of using it, that you can now proceed with unsubscribing from that.
In other words: put your card in, set it up, send some mail, try it out, check that email sending works and all that jazz - and after that only then downgrade.
Thank you! I wish their pricing model wasn’t so deceptive, they hide the “flex mode”