Self-Employment help??

Hi. I have a Small Earnings Exemption Certificate from HMRC so I don't pay UK tax. My net earnings are slightly over the USA $400 boundary line for self-employment, but as I was reading all the forms for USA taxes, I became confused. My sales were from my novel. Some sold in the USA, some in the UK.  I'm guessing the copies which sold in the UK will be my foreign earned income. I read that it can be excluded from the gross income; however, I'm supposed to pay taxes on all income (USA and UK for the IRS).  So what's the point in excluding foreign income from the gross, if I'm expected to pay taxes on it?   I don't have money for a taxpreparer to help me. We are currently selling off things in the house just to pay the electric bills and car insurance, as I am disabled.  Thank you, if anyone is able to help me with information.

You are required to file a tax return if you are a US citizen and your income exceeds the applicable filing threshold; which it does. Based on income of just over $400 you would owe no tax even if you filed.

Guya, yes I know I have to file my forms anyway. I'm relieved I probably won't have to pay any tax. Thank you so much for replying!