La. Revenue and Taxation § 47:1682
Refunds of such taxes shall be made only when the claim therefor is supported by special invoices issued by dealers who have obtained special permits pursuant to R.S. 47:1683, and only when the claim therefor is supported by a certificate setting forth that the claimant has used the gasoline for a refundable purpose, together with a sellers invoice. The invoices must have been paid by the claimants and marked paid by dealers and shall be on invoice forms prescribed by the collector of revenue. Claims for refunds must be filed with the collector within six months after the date of the purchase. Amounts refunded shall bear interest at the rate of six percent per annum commencing ninety days after the claim for refund in full compliance with this Part is received by the collector of revenue.
La. Revenue and Taxation § 47:1682