For simple transactional e-mails, I am just using server-side rendering of React components. Since these mails are not that complex, this proved to be the easiest option. When I was still on Blaze, I was using some server-side Blaze rendering package from meteorhacks / Arunoda.
Another great and very cost-effective option is Mautic in combination with AWS SES. The SES prices are ridiculously low, like magnitudes smaller than any other e-mail service I am aware of. It’s literally cents (or even free) versus hundreds of dollars. I can highly recommend this combo, especially if you don’t just use it for transactional mails, but also for newsletters and re-engagement activities. We couldn’t be happier with it.
It’s also super-simple to add new user emails to Mautic via their API (e.g. as newsletter recipients), as soon as they register to your Meteor server. The whole process (installation, SES connectivity, Meteor connectivity) took me about half a day, maybe one if I count in some additional experiments.