(a) 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.

(b) 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 an income tax 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.