Postcards are restricted in size, etc.: address must be parallel to the long dimension. Minimum 3.5" x 5" and maximum 4.25" x 6". If making your own at home, use a minimum 65-pound card stock. Most commercial postcards and all USPS stamped cards meet this thickness and dimensions.
Postcards you make or buy or have printed commercially cost $.35 each to mail. Stamped cards are sold by the U.S. Postal Service for $.39 each and if your local Post Office doesn’t have any in stock, there are at least two varieties available via their website usps.com at their postal store. On the website, choose "cards and envelopes," then "postcards." USPS will ship them to you for a minor mailing charge in addition to the card charge.
MAKE YOUR OWN ~ See this card mailed from NC to NH recently:
See one of the current postcard stamps below and an image of a postcard message you can use to contact your federal elected officials to Save the Post Office -
SAVE THE POST OFFICE Postcard limited time offer ~ Mail a self-addressed stamped envelope (preferably #10) to us and we will return 3 preprinted "SAVE THE POST OFFICE" postcards (shown above) that you can use to mail to your 2 U.S. Senators and your Congressional Representative. See below for our mailing address. Offer good through August 25, 2020. Feel free to copy and save the image above and print your own on cardstock. Our cards do not have postage or addresses.