Dimensions: Between 3-1/2 by 5 inches and 6-1/8 by 11-1/2 inches. To qualify for the card price, cards must be between 3–1/2 by 5 inches and 4-1/4 by 6 inches. Larger postcard sizes are mailable, but they are charged at the regular First-Class Mail letter price. Card thickness must be between 0.007 and 0.016 inch.
If letter mail is more than 4–1/4 inches high or more than 6 inches long, it should be at least 0.009 inch thick.
A surcharge is assessed for nonmachinable letter-size mailpieces (101.1.2). The surcharge must be included in the prepaid postage amount.
Ink/paper Colors and Type Styles: Not all colors of paper and/or ink and type styles are compatible with automated equipment. Contact your local Post Office for guidance.