The letter you received is the notice, the collection action could include the levy on the non-employment payments.
The IRS generally has ten years to collect taxes once it has been assessed. The term “assessment” simply means the IRS has determined that a tax is due.
The IRS can levy both income and property including wages, payments from customers, retirement income, social security payments, state and federal tax refunds, savings and checking accounts, vehicles, real estate, or other valuables.
You generally have 30 days once the notice is sent to make payment, enter into an installment agreement, or attempt an offer-in-compromise.
A non-continuous levy is generally brought against someone who receives a form 1099-MISC, and is considered self-employed.
If the [payment they are asking for is too high you can request a lower payment amount by showing your income for the period and requesting a lower amount until the slow season is past.
If you just received the notice then act quickly to request they reconsider the payment.