How savy are you?
Create a gmail account for your app
Go to script.google.com
Sign in as your newly created app instead of yourself
Then ship out some short js and publish the script as a webapp
When you publish, you’ll get a url, paste that into your apps api connector
Now you can send html formatted emails
If you continue and want to read their docs or snoop around stackexchange, you can also send the ics as a url attachment
So if how you make them can produce a url, you can the. Email that url. All free and as reliable as gmail can be.
I believe you can send thousands per day for free this way.
I only send formatted emails with online images and texts. I’ve never sent an attachment, I just know it’s possible and a lowly explored option here due to the minor amount of code and research needed. Probably less convinient too.