Applicability of the New Rules. The new cargo securement rules apply to the same types of vehicles and cargo as the old rules, covering all cargo-carrying commercial motor vehicles (as defined in 49 CFR 390.5) operated in interstate commerce.
Most commercially-produced postcards measure approximately 4.21 inches by 5.47 inches. Less commonly, smaller postcards are typically 3.5 inches by 5.5 inches, close to the minimum size the USPS allows.
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.