How much postage must be placed on a postcard mailed within the US?

postcard_must_be">The price of a single First-Class postcard stamp is $0.29 To qualify for mailing at First-Class Mail® prices, a postcard must be

• Rectangular and have angular (90 degree) corners
• At least 3- inches high x 5 inches long x .007 inch thick
• No more than 4-1/4 inches high x 6 inches long x .016 inches thick