How can a taxpayer confirm that the IRS has received a payment, other than finding out that they owe penalties and interest somewhere down the line?
Assume the payment instrument (money order) is prepaid and would not debit an account like an ACH, check, debit, or credit transaction would.
Researching more, would any variation of Form 4506/-T/-EZ help on this schedule?
It would list the information the IRS received for the tax return (at least confirming that they received it) although it doesn't specify that it would indicate whether or not a payment has been remitted. This could be relevant if the payment were sent with the return but does not solve the problem if the return were sent separately from the payment or if the return were e-filed and payment mailed separately.