@GeorgeKa
If you have accurate records of your expenses, and if you can get your taxes in order before the expiration date on the notice, then it would be a better route to prepare an amended 2021 tax return which reports the self-employment income as well as the expenses on schedule C and schedule SE. The expenses will reduce the SE tax that you owe. Then, send a letter to the office that sent you the notice in which you acknowledge that you made a mistake and you should have reported self-employment income, but that with the inclusion of your expenses, the SE tax that you owe is less than what they billed you for. Include the amended return and payment for the adjusted amount of tax. (Mail the amended return to the office along with your letter, not to the normal address for amended tax returns.)
If you pay the entire amount from the notice, and then try to file an amended tax return to adjust it, that paperwork becomes much more complicated.
