Postage was a fee levied by a mail delivery agency for use of their services. This money was typically paid prior to delivery of the parcel.
On Sigma Iotia II, Bela Okmyx once returned a letter from one of the residents of his territory with "postage due," indicating that he had not paid to send the letter back. (TOS: "A Piece of the Action")
Payment was often indicated by a postage stamp, a piece of paper affixed to the corner of an envelope. They were wet by saliva using one's tongue. In 2000, Shannon O'Donnell offered to help Jason Janeway and his father Henry with their campaign against the Millennium Gate. Jason assured her that "you don't want to be licking stamps for the next two days", as his dad didn't have a computer from which to e-mail those they wished to write letters to. (VOY: "11:59")