Say you are building a custom integration with Klaviyo, and you need to send transactional order data from your site into Klaviyo. Which type of API should you use to send this data?

When handling transactional data—such as order confirmations, shipping updates, or password resets—reliability and security are the top priorities. Why Server-Side is the Industry Standard Transactional data is "mission-critical." If a customer buys a product but never receives an email confirmation because their browser blocked a script, it creates a poor user experience and increases support tickets.