No register forms, not another password to forget, not another account to manage.
You don't need to learn/develop another substitution/merge tag's set. Use {{handlebars expressions}} for your templates.
Just a POST request with a JSON payload and you're done!
Use postman, curl or your own project to make a POST request to your ´s webhook with the JSON payload for your template. That´s all!
You can now test your transactional emails using a NON-production environment.
Last but not least, you don't have to pay a cent!
We know, we thought the same, but.. just think about it:
Don't you think there is a better way? Did we get used to this problem?
First, accepting that we had underestimated the problem for 16 years, then:
Just make the math, how much wasted time on all the email stuff? Time is gold. Maybe you are paying an expensive developer to spend time solving the same problem in every project.
Let's see what Wikipedia says about it.