Not a good look questioning people who know what they are doing by promoting a service apparently built by someone who does NOT know what they are doing.
I just open-sourced a fully customizable payment update page micro-service for Stripe .
It has a small footprint (~8Mb of memory) and is already available as a docker container, launch it and you are good to go !
We built our own library to manage transactional email, you define an HTML and Text template, a meta file and then the library inject your data and automatically inline css, it's for NodeJS but you might take some idea from it
https://github.com/FGRibreau/node-transacemail