Postcards are classified by their size and thickness. The benefit of mailing postcards is price and that the customer can see everything without having to open it.
Letters
Mail classified as a letter is either mailed in an envelope or a larger piece of paper folded down to letter size. 3.3oz maximum weight
Flats (Nonletters)
Magazines, Newsletters and Large Envelopes fall into the Flats Category. Flats are classified by their size, weight and thickness. Flats must weight no more than 16 ounces.