Taxpayer Penalty Protection Act of 2018 This bill amends the Internal Revenue Code to establish a safe harbor to reduce the amount of estimated income tax that certain taxpayers are required to pay for 2018. (Under current law, a penalty applies if taxpayers do not use either withholding or estimated tax payments to make a required annual payment before the end of the year. The required payment is the lesser of 90% of the tax for the taxable year or a specified percentage of the preceding year's tax.) For 2018, the bill reduces the percentage of the preceding year's tax from 100% to 80%. For taxpayers with adjusted gross incomes for the preceding year that exceed $150,000, the bill reduces the percentage from 110% to 100%.