Interest and penalties are paid by TurboTax for a calculation error in the software but not any additional taxes owed.
See this TurboTax support FAQ for the calculation guarantee - https://ttlc.intuit.com/turbotax-support/en-us/help-article/account-management/turbotax-100-accurate...
Contact TurboTax Audit Support Center for assistance with the IRS letter you received - https://support.turbotax.intuit.com/irs-notice/audit-support/
