39 CFR 551.6 - Pricing.
(a) The Semipostal Authorization Act, as amended by Public Law 107-67, section 652, 115 Stat. 514 (2001), prescribes that the price of a semipostal stamp is the rate of postage that would otherwise regularly apply, plus a differential of not less than 15 percent. The price of a semipostal stamp shall be an amount that is evenly divisible by five. For purposes of this provision, the First-Class Mail® single-piece stamped first-ounce rate of postage will be considered the rate of postage that would otherwise regularly apply.