The tax paid to a vendor in this State may be applied by the purchaser as a credit against the tax due from the purchaser on all fuel used in this State in the reporting period in which the fuel, with respect to which the tax was paid to the vendor, was used.

The amount of credit allowable is the amount of tax separately stated or included in the selling price on the receipts (invoices) issued by the vendor to the purchaser for purchases of fuel delivered into vehicle fuel tanks. No tax credit may be taken for unauthorized payments of the tax on fuel delivered into a storage facility other than a vehicle fuel tank. To be entitled to the credit, a purchaser shall retain for inspection by the Board all receipts (invoices) given by vendors showing the amount of tax paid or included in the selling price, together with evidence of payment.

If within a reasonable period of time the purchaser has accumulated surplus credits which have not been applied to payment of tax liability under Section 8651 of the Revenue and Taxation Code, or if the purchaser ceases to be a user in this State, a claim for refund as provided in Sections 9152 and 9153 of said code should be filed. All claims for refund of overpayments shall be accompanied by the receipts (invoices) obtained by the purchaser from the vendor and evidence of payment.