(1) An inactive vested member may purchase any additional service for which the member is eligible any time prior to retirement. The appropriate statutes and rules will be followed to calculate the cost to purchase the service except the inactive member's last termination date will be considered the purchase request date. Interest at an effective annual rate of 8% per year, compounded monthly, will be charged from when the member last terminated to when the member completes payment for the cost of the purchase.