1. The determination of the amount of tax due shall be the monthly gross retail prescription receipts reported to the department of revenue multiplied by the tax rate established by rule by the department of social services. Such tax rate may be a graduated rate based on gross retail prescription receipts and shall not exceed a rate of six percent per annum of gross retail prescription receipts; provided, that such rate shall not exceed one-tenth of one percent per annum in the case of licensed pharmacies of which eighty percent or more of such gross receipts are attributable to prescription drugs that are delivered directly to the patient via common carrier, by mail, or a courier service.

2. The department of social services shall notify each licensed retail pharmacy of the amount of tax due. Such amount may be paid in increments over the balance of the assessment period.

3. The department of social services may adjust the tax rate quarterly on a prospective basis. The department of social services may adjust more frequently for individual providers if there is a substantial and statistically significant change in their pharmacy sales characteristics. The department of social services may define such adjustment criteria by rule.