You are working as an independent contractor. The IRS considers you to be self-employed. Grubhub, Instacart, DoorDash, etc. are not going to withhold tax from your pay so you will have to pay self-employment tax for Social Security and Medicare as well as ordinary income tax.
When you file your tax return you will have to prepare a Schedule C to show your business expenses such as mileage. You will need to use any version of the CD/download software or the online Self-Employed software.
You need to be keeping good records yourself of your expenses such as mileage on your car.
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