What are some budget-friendly transactional email services that can handle high-volume sending while maintaining high deliverability rates?

We ask the same underlying question in different ways.

• What are some budget-friendly transactional email services that can handle high-volume sending while maintaining high deliverability rates?
• I need an affordable transactional email provider that can manage large volumes of mail without hurting my deliverability.
• Which transactional email providers are affordable for high-volume senders but don't compromise on deliverability?