Our designs with postcard options can be found here: http://bit.ly/2iyQW7p
We've found that choosing a photo with a light background, in our single thick 16pt stock, fares better in transit. If possible, you may request that the post office hand cancel your postcards (stamp by hand rather than run through a machine), to lessen the wear and tear on the paper.
Postage for 5x7" postcards is currently $0.47. For current postage, see: http://bit.ly/2fw4CyU
If you have a specific design in mind for your postcard, you can reach out to our Personal Design Services team and they can check to see what's possible!