this is from an IRS website
Before contacting the IRS, first check with your financial institution to verify whether the check has cleared your account.
If it's been at least two weeks since you sent the payment to the IRS and your financial institution verifies that the check hasn't cleared your account, call the IRS at 800-829-1040 to ask if the payment has been credited to your tax account.
If the payment hasn't been credited and your check hasn't cleared, you may choose to place a stop payment order on the original check and send another payment. If you choose this option, the IRS won't charge a dishonored check penalty. And you may be reimbursed for bank charges related to stopping payment. See the Form 8546, Claim for Reimbursement of Bank Charges PDF for more information on claiming reimbursement of bank charges.
https://www.irs.gov/pub/irs-pdf/f8546.pdf
lost in the mail does not seem to meet the requirements.
in the future, I would advise using direct pay. when you finish and submit the request you cam print out the confirmation.
https://www.irs.gov/payments/direct-pay-with-bank-account
if you prefer the uSPS, then you should use a tracking service so you know it got there.
