Red River Paper has greeting card stock of 10.5 x5.25 that is 5.25 x 5.25 folded. The square format gives you extra possibilities in cropping and adding corner flourishes (see example).
My greeting card online class shows you many ways to take photographs especially for cards, how to work with text, and Photoshop techniques. http://www.esunrisedesigns.com for details
When sending a square card, I use the 5.25 x 7.25 envelope which is normal postage. If you use square envelopes, there is a 20 cent surcharge as they do not go through the USPS machine. At least that is my understanding of the surcharge.