I also received the same email. I contacted YNAB, and received this message back:
"Ack! Sorry for the scare here!
First, I want to confirm for you that this email is not from Apple or us and you will not be charged.
Earlier this week we learned that a phishing email was going around claiming to be from Apple saying you would be charged for a YNAB subscription. We did some investigating and we're able to confirm that the email isn't targeting YNAB users and our systems have not been compromised. This is a phishing attack using our name to try and gain access to people's Apple accounts.
Apple has a system in place to try and stop bad actors like this. You can help by sending the email you received directly to Apple. All you need to do is forward it to Apple at reportphishing@apple.com so they can take action. He put together a quick Help Doc with a few more details and resources if you're interested.
Thank you for helping us stop these kinds of attacks!
If you have any questions at all (about this or anything else YNAB related), please let us know!"
Hope this helps!