(a) As used in this Code section, the term:

(1) "Final return" means the original income tax return filed by the taxpayer for the tax year or an amended return filed on or before the due date of the return without extensions. Such term does not include any other amended income tax return for the period or an estimated tax return.

(2) "Income tax liability for a taxable year" means the taxpayer's income tax liability as calculated under Code Section 48-7-20 or 48-7-21 for the taxable year reduced (but not below zero) by all nonrefundable credits to which the taxpayer is entitled. Nonrefundable credits include any credit that is limited by the taxpayer's income tax liability or some percentage thereof.

(3) "Other credits allowed by law" means only those income tax credits that are refundable, such as the credit for income tax withholding and the credit allowed by Code Section 48-7-28.1. Refundable credits do not include any credit that is limited by the taxpayer's income tax liability or some percentage thereof.

(b) The amount of estimated tax paid under this article for any taxable year shall be allowed as a credit to the taxpayer against the taxpayer's income tax liability under Code Section 48-7-20 or 48-7-21 for the taxable year.

(c) To the extent that the estimated tax credit, together with other credits allowed by law, is in excess of the taxpayer's income tax liability for a taxable year as shown on a final return filed by the taxpayer for that year, the overpayment shall be considered as taxes erroneously paid and shall be credited or refunded as provided in this subsection. The overpayment shall be credited to the taxpayer's estimated income tax liability for the succeeding taxable year unless the taxpayer claims a refund for the overpayment. The commissioner may consider any final return showing an overpayment as a claim for refund per se. An overpayment shall bear no interest if credit is given for the overpayment. Amounts refunded as overpayments shall bear interest at the rate provided in Code Section 48-2-35 but only after 90 days from the filing date of the final return showing the overpayment or 90 days from the due date of the final return, whichever is later.