This CSR discusses the standards for mailing a postcard at the First-Class Mail card prices.
A question was raised as to whether a folded sheet of paper (with one half of it glued to the other half along the edges) can be mailed at the prices prescribed for postcards.
Prices for postcards are lower than those for letters to reflect the requirements that these cards conform to all of the prescribed physical restrictions. Domestic Mail Manual (DMM) 18.104.22.168*, provides that postcards must be rectangular in shape. Such postcard must be at least 0.007 inch thick and not more than 0.016 inch thick.
DMM 22.214.171.124 prescribes (i.e., a card not supplied by the USPS) must be of uniform thickness and made of unfolded and uncreased paper or card stock of approximately the quality and weight of a stamped card. It may be formed of one piece of paper or card stock or two pieces of paper that are permanently and uniformly bonded together. The stock used for a postcard may be of any light color that does not prevent legible addresses and postmarks from being placed thereon. Brilliant colors must not be used.
The postal regulations governing postcards contained in DMM 201.1.2 are based on statutory law enacted by the Congress (Act of May 19, 1898, Chap. 347, 30 Stat. 419). Historically, there have been rather precise statutory restrictions to which postcards have had to conform. From their beginning in 1898 as private mailing cards, Congress mandated that cards "... be approximately of the same form, quality, and weight as the stamped postal card ...." Quite simply, it is not enough for a customer to create a product and represent it as a postcard merely because it fits within the size restrictions prescribed for a postcard. A postcard must necessarily conform to all other requirements in DMM 201.1.2, including being of approximately the same form, quality, and weight as a stamped card. When a piece of paper has been folded over and glued together on the edges to create a peel-open mailpiece, that mailpiece cannot be said to be representative of the same form as a postal card because stamped cards are not formed that way. The postcard "form" restriction can be no more overlooked than any other physical restriction legally mandated for postcards.
*See also DMM 101.6.2.