If $13,500 net self-employment income is your only income for the year, your standard deduction of $12,550 wipes out most of your income. You also get a deduction for half of your self-employment tax, and you might have a small QBI deduction, so you end up with no taxable income. Look at the summary that Critter-3 posted. Note that is shows taxable income of $0.00. The only tax you are paying is self-employment tax. That's the $1,900 of federal tax. And a large part of that gets eliminated by the Earned Income Credit (EIC). That's what the $1,370 of credits is.
