"You may think that your mail piece is a ‘card’, because it is a single sheet of paper. But to qualify for mailing at First-Class Mail Card rates, a card must be:
• Rectangular
• At least 3-1⁄2 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
If your mail piece does not meet the dimensions above, then the Postal Service considers it a letter (charging letter rate)."