If you did not receive a Form 1099-MISC for this income, report it in the "Other Reportable income" interview of TurboTax.This avoids the need to use either the 1099-MISC interview or the Business Income features of higher versions of TurboTax. 

Go to the Other Reportable Income  interview, the very last interview in Wages and Income (Federal Taxes tab > Wages & Income > Less Common Income > Miscellaneous Income).  For the Description, enter "Postmates Receipts".  

 (Or enter "other reportable income" in the Search Box and select "jump to other reportable income". These payments are NOT wages).  

This income will appear on Line 8, Form 1040.