You are going to need help. Professional help would be best. An attorney or an accountant is a wise move.
You need to start looking for receipts.. lots of them. Did you use online shipping for your packages? If so you should be able to find that information. You also should look into yearly paypal reports. That is a good place to look for expenses. Also check your credit card statements or bank statements. You must have some records of expenses. You should be able to get copies of statements if you have not kept them.
You are entitled to deduct for expenses: For example, shipping costs, fees, cost of goods, cost of supplies. There are lots of expenses that sellers typically show on schedule C of the tax form.
You will probably need to provide proof of those expenses and they might take your word for it at this point. You can deduct what you can prove.
And before you get another letter, you might want to take care of 2018 income tax if you haven't yet. The same will apply. But those are questions and things you need to work out with a professional. No one here can advise you on the specifics....
And start pulling together 2019 records as you will need those as well to file your taxes properly.
It is a good wake up call to others that think that they can sell online and not report their income.
Keep every receipt, claim every expense, claim every bit of income. You will sleep better at night.
