Postcards are mailers that do not have an envelope to open, according to the United States Postal Service's (USPS) website. Depending on the mail service you are using, the cost and requirements for a postcard vary.
The USPS considers a postcard to be any single rectangular piece of paper that is "at least 3 1/2 inches high by 5 inches long by 0.007 inch thick'" and "no more than 4-1/4 inches high by 6 inches long by 0.016 inches thick." The post office considers all papers outside of these dimensions to be letters.
Postcards are mailed as First-Class Mail or Standard Mail. Postcards that are mailed as First-Class Mail receive special postcard pricing, but must meet the USPS size requirements and attachments such as magnets or stickers may disqualify them from the postcard mailer rate. Postcards that are mailed as Standard Mail do not receive their own postage price, and are charged at letter prices.
Postcards that are mailed as First-Class mail are able to be placed in any USPS collection box. According to the USPS, mailing a postcard as First-Class Mail is the most economical choice for most customers. Postcards that do not meet the size requirements are still able to be mailed, but additional postage is typically required.