Resend, at $20/mo against $100/mo.
Customer.io vs Resend
Resend is cheaper at $20/mo, against $100/mo.
Lifecycle and CRM automation across email, SMS, push

Developer-first transactional and broadcast email
Customer.io and Resend are alternatives in the Lifecycle email category. Customer.io: Lifecycle and CRM automation across email, SMS, push. Resend: Developer-first transactional and broadcast email. Resend is cheaper at $20/mo against $100/mo.
Lifecycle and CRM automation across email, SMS, push. It sits in the lifecycle email category.
Developer-first transactional and broadcast email. It sits in the lifecycle email category.
They are alternatives at the same step, so most stacks pick one or the other. No aistack.sh stack uses both today, though nothing prevents a custom workflow.
Beyond each other, Customer.io's closest alternatives are Klaviyo. Resend's closest alternatives are Klaviyo.
