Semipostal Stamp Clarification Act of 2021 This bill removes the authority of the U.S. Postal Service (USPS) to issue semipostal stamps (stamps sold at a premium to raise funds for causes of national public interest) except as provided by an act of Congress. (Currently, the USPS issues semipostals using its own discretion to determine the causes it considers to be in the national public interest.) The bill also makes entities (currently, only agencies) eligible to receive amounts that become available from the sale of semipostals. Congress must determine the appropriate agencies or entities that receive such amounts. The USPS must issue regulations addressing how administrative costs incurred relating to semipostals shall be computed, recovered, and kept to a minimum.