The most common IRS audits are done through the mail where the IRS sends you a letter and asks you to mail in verification of certain items from your tax return. These ‘correspondence audits’ are generally handled through the mail to request missing documents needed to complete your tax return, like W-2 and 1099 interest expense or income items.
It is important that you respond to these mail requests because if you do not, the IRS will either deny the deduction in question, or increase your gross wages, which will lead to you having to pay more in taxes.