We are building an API for sending real greeting cards (5x7 closed, 10x7 open) and also considering doing a Bubble plugin. Our service currently exists as a Slack integration for teams - cardybot.com.
It will give you the ability to:
-Use your own card front or one from our stock
-Provide a single or group (like the ones on our site) messages for the inside. Emojis, fonts and colors included.
-And off course, mail globally
If this is something your bubble app can use let me know or DM me so we can plan to do it!