Papers Available In Square Format:
60lb. Polar Matte Single Sided - The best photo matte available today
60lb. Pecos River Gloss - Glossy on one side, plain paper on the back
They fit Special High Quality White Envelopes available in 100 or 250 packs.
A special note about postage: Square items cost a little extra to mail. As a general rule, a square card will require postage for one ounce more than the actual weight. The USPS sells a Special Stamp for Square Cards which offers the convenience of paying for a 1-ounce square card.
Red River's square cards are 10.5" x 5.25" and fold to 5.25" x 5.25".