If you filed the instacart on a 1099 and Schedule C you owe self employment tax on it. You need to have more taken out of Walmart or send in quarterly estimated payments to cover the extra tax.
Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.
