The postage required for postcards is generally less than postage required for standard letters; however, certain technicalities such as their being oversized or having cut-outs may result in payment of the first-class rate being required.
There ain't any news in being good. You might write the doings of all the convents of the world on the back of a postage stamp, and have room to spare. Finley Peter Dunne
Worldwide the most common method of prepaying postage is by buying an adhesive postage stamp to be applied to the envelope before mailing; a much less common method is to use a postage-prepaid envelope