Thank you for contacting us.
Point is that we can’t include the third-party plugin without the permission of the plugin author. We don’t have such permission from the plugin that you mentioned. So, you can use it only by buying a personal plugin license.
About Subscriptio plugin emails. Unfortunately, this plugin does not support email builder, issue discussed with other customer here https://www.8theme.com/topic/subscriptio-custom-email-builder-emails-are-missing-from-options/#post-323088
So, don’t create email templates for the Subscriptio plugin emails using email builder, use the default emails of that plugin.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.