If you're planning to send as a postcard, where are you planning to put the receiver's address and your return address? It seems they're not designed for that.
Many of my sons' friends and one of my sons have sent Save The Date post cards in the past few years and all of them have been sent without an envelope. So, they should be fine. If any are being mailed outside of the USA, then using an envelope would make more sense. But, again, you need to redesign the card a bit to make it easier for the Post Office to identify the address.
Also, if this is a photo of the actual card you're sending, are you paying for the time machine to send everyone back to 2012?

_________________
Sandy McCauley
Manager of Customer Relations and Education for KNK USA
Owner of
http://www.iloveknk.com/