I have not used Nylas but if email is a tool that is integral to your productivity, paying for a tool that helps you is not a far-fetched idea. It's the same as with text editors. There are plenty available for free but if there's one that happens to help you in your work, paying less than 0.1% of your income can hardly qualify as excessive.
At $7/month, if this tool saves you 10 minutes a month, it's worth it.