As of April 2014, it costs $1.15 to send a postcard from the United States to France. This is for a standard-sized postcard, 6 inches long and 4.25 inches wide.
Letters are less standardized. Their cost varies according to their weight. For instance, a 2-ounce letter costs $2.13 to mail to France. Whether a customer is mailing a postcard, a letter or a package to France, there are both maximum and minimum size limitations. For instance, a postcard is allowed to be no longer than 6 inches. The U.S. Post Office sends international packages and letters inside USPS Priority Mail envelopes and boxes, as well as non-USPS containers.