A producer who has sold sheep and/or wool and has had an assessment deducted from the sale may, by application in writing to the board, secure a refund in the amount deducted. The refund shall be payable only when the application has been made to the board within 60 days after the deduction. Interest shall be allowed and paid at the rate of 6% per annum upon the total amount of such assessment imposed by this Act, except that if any such assessment is refunded within 90 days after an application for refund has been made within the required 60 days after deduction or within 90 days after the first purchaser of sheep and/or wool remits the assessments withheld and deducted to the sheep and wool production development and marketing board, whichever is later, no interest shall be allowed on such assessment. Each application for refund by a producer shall have attached thereto proof of assessment deducted.