Small Business Owners' Tax Simplification Act of 201 9 This bill modifies several tax-related requirements that affect small businesses and self-employed individuals. The bill includes provisions that align the deadlines for quarterly estimated tax payments with the calendar year quarters; modify the dollar thresholds for various information reporting requirements; allow certain self-employed individuals to participate in cafeteria benefit plans; exclude from self-employment income net earnings that are less than the amount required under the Social Security Act for a quarter of coverage for the calendar year in which the tax year began; allow certain health insurance costs of self-employed individuals to be deducted for self-employment tax purposes; and specify that voluntary tax withholding agreements, training, or group discount programs have no effect on whether an individual is classified as an employee or an employer. The Department of the Treasury must (1) establish uniform standards and procedures for the acceptance of digital or electronic signatures, and (2) use prenotification testing to verify recipient information before transferring a tax refund or credit through an electronic funds transfer.