Postage is the cost of sending something in the mail. If the postage for mailing your brother's collection of porcelain pigs is too high, you might want to bring them with you when you drive to his house instead.
Use the noun postage for the expense of mailing letters and packages, or for the stamps themselves: "Don't forget to attach the postage before you put that in the mailbox!" Some people, in fact, refer to them as "postage stamps." Postage comes from post, "mail system": early mail carriers were "posted" along a road, like a relay race, to speed delivery.