Information related to transactional emails:
There are two types of email flows that can be setup in Klaviyo: Marketing and Transactional Emails. Klaviyo is mainly used for marketing emails but you can also send transaction emails, only you needs to keep some key details in mind:
To send transactional emails with Klaviyo, you must first contact Klaviyo Customer Support team. Transactional email requests are manually reviewed and approved by Klaviyo.
Klaviyo's policy (read full policy here) prohibits any marketing content in transactional emails and according to this policy, businesses are not allowed send marketing emails without an unsubscribe button.
Sending marketing content to unsubscribed recipients via a transactional email might violate the CAN-SPAM Act. However, Klaviyo can change flows into transactional emails that will be sent to everyone (even if customers have unsubscribed marketing emails), and such emails don't require an unsubscribe button.
Klaviyo is also strict with scrutiny of emails that there must not be marketing content and if a business breaks this policy the penalty could be account suspension.