39 CFR §551.6

1. (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.
2. (b)The prices of semipostal stamps are determined by the Governors of the United States Postal Service in accordance with the requirements of 39 U.S.C. 416.