Gumroad doesn't even send 1099's, like Amazon does for Kindle authors. I am not an accountant, but I believe it's just gross receipts on your Schedule C.
If you make over $400 in income from book sales you need to take that income and apply self-employment tax on that income.
In addition you need to make quarterly payments for the estimated tax.
If you haven't done so yet. To fix your situation you can file an amended return. A CPA can do it for you... given your income, you should be able to afford it.
As your income grows so it does the likeliness of being audited.
Thanks! Yeah I mean it's not that different from my freelancing. I've been paying estimated tax and self-employment tax for years. Mostly now it's about selling products (example: I have to deal with New Mexico gross receipts tax, ugh) vs services.
You only need to make quarterly payments if you meet certain criteria, largely if the revenue is above a certain amount and you have a reasonable expectation the revenue will continue.
