If you are a U.S. citizen or resident alien, the rules for filing income say that you are subject to tax on worldwide income from all sources and must report all taxable income and pay taxes on that income.
What is the nature of the income that did not reach the level of $600? The nature of the income will determine where and how the income is posted.
You need to claim it as Self-Employed income. If using Turbo Tax online.
If you use Turbo Tax Desktop, the steps are similar.