Have I got a quick tip for you!
Use CD or DVD mailing boxes depending on the height & width of your card. Just wrap your card in a lovely coordinating tissue paper, put a bow on it or tie a ribbon on it & slide it into the CD or DVD mailing box and it is ready to go.
Another option would be to use bubble mailers. Since they already have the bubble wrap lining in the envelope you would only need to place the card gently into the envelope.
Don't forget to mark it FRAGILE.
Hope this helps!
Have a great crafty day.