Mo. Rev. Stat. § 376.980

Each pool member exempt from chapter 148 shall be allowed to offset against any sales or use tax on purchases due, paid, or payable in the calendar year in which such assessments are made. Further, such assessment, for any fiscal year, shall not exceed one percent of nongroup premium income, exclusive of Medicare supplement programs, received in the previous year. If the assessment exceeds the part of any sales tax or use tax due or payable in such year, the excess shall be a credit or offset carried forward against the part of any sales tax or use tax due or payable in succeeding years until the excess is exhausted. The director of revenue, in consultation with the board, shall promulgate and enforce reasonable rules and regulations and prescribe forms for the administration and enforcement of this law.