They will most likely need extra postage. The USPS charges extra for various things such as weight (a regular Forever stamp covers up to 1 ounce only), size, how thick the envelope is, the shape (square envelopes require additional postage even if it’s less than 1 ounce) and some other things. There is a postage calculator on the USPS site that can help you figure out how much postage to put on your cards.