Sec. 7.5. "Postage charges", for purposes of this article, mean amounts that:
(1) are for the purchase price of stamps or similar charges for mail or parcel delivery through the United States mail, without any additional amounts added to the actual price; and
(2) are incurred by a seller on behalf of its customers or purchasers.
Postage charges do not include any charges for mail or parcel delivery by any means other than through United States mail.