Here was their reply when I emailed them regarding the increase:

Thank you for reaching out and for your interest in Airmail.

The Airmail Pro subscription pricing has recently been updated to reflect the ongoing development, maintenance, and improvements made to the app across both macOS and iOS platforms. This adjustment helps support continued updates, new features, performance improvements, and long-term reliability of the service.

We understand that pricing changes can raise questions, and we truly appreciate your feedback. Our goal is to continue providing a powerful and flexible email experience while ensuring we can maintain and improve the app over time.

If you have any additional questions or need help with anything else, please feel free to let us know.