I see you specifically added "by mail" to your question. Still, (a) given the nature of the IRS' COVID-related delays ANY paper filing with the IRS should be avoided if at all possible, (b) the 1040-X no longer has to be filed on paper as of last tax season (as it was for many years before), and (c) a cover letter may still confuse the IRS when TurboTax should automatically generate the statements, schedules, etc. to be included with a paper 1040-X.
