Each post card is quilted on the front, contains a stiff middle "batting", and a piece of card stock on the back so you can write a note. And, yes, they can be sent through the mail. I usually take mine to the post office just to make sure I've got the right postage - usually around $.70. Cards are 4" x 6".
Please look at the third photo carefully. The front of the card is covered with tulle. You may or may not like that.