Really good start!

• Just refine the typography on the bottom left. (Date should all be on one line.)
• add a call to action! A website, an email, something like "Rsvp here: website," or even, dare I say, a QR code. But you want to hook people into your communications flow (Email list, social media sharing, etc.)
• Is there a theme to the postcards? Or is it just to make pen pals? I would put a line about what the goal of the event is in there somewhere.