Did you tell them you're self employed and ask them what you can show them instead? Seems like they'd be prepared for this to sometimes happen rather than asking people to falsify documents More on reddit.com

You can run as a Sole Prop without an LLC, or you could go ahead and add the LLC for liability protection if you feel you need it. In either case, you simply pay yourself an "owner draw", and this would show up on a schedule C on your tax return. If you get to the point where you're netting ~$45k/year in profit through this business, then it would be in your best interest to consider filing an S-election, which means the LLC would be taxed like an s-corp. In this case you have an actual payroll system, and you cut yourself actual payroll checks of a "reasonable salary." Any additional amount you pay yourself above that would be a distribution. Distributions are not subject to Self Employment taxes, so this saves you a bunch of cash. If you aren't at that point yet, though, there's really no need to run a payroll system and give yourself "paychecks". Just take owner draws. As you said, you'll invoice your client(s) and they'll simply pay the invoice. That payment should go into a business account of yours. Pay all your business expenses out of this account, and hold enough aside for tax that you will owe. The rest you give yourself as a draw. Literally a transfer from the business account to your personal account. More on reddit.com

Without the letter number, I'm guessing the IRS wants to verify that your wage and withholding information is correct. It's possible one of your employers did not send the IRS your W2, or you reported information different from what the employer reported to the IRS, so the IRS is asking about the discrepancy.

The IRS did not ask for self-employment information so don't send that. However, be sure to keep good records of your self-employment income and expenses in case they are requested in the future.

More on reddit.com

you have to pay taxes even if individual jobs don’t have to issue 1099s. Just making under $600 per job (the 1099 threshold is $600 not $400) doesn’t mean you don’t owe taxes on the income. You should amend your past 3 years of tax returns to properly reflect your work, which will likely be reported on a schedule C. More on reddit.com