Filed 2024 taxes online; mailed check & voucher; Tracking says IRS got it; BUT check not cashed & IRS letter says I'm not paid & they added $10 penalty. What should I do?

[Flying-LP]

I am willing to pay online and absorb the penalty just to clear everything up; but am concerned that IRS also has my check. After I pay, somebody may "find" the "lost" check and cash it. Then what?

[taxlady28]

There is no voucher to mail with your check to pay your taxes.  I believe you mailed a voucher that is used to make a payment towards your 2025 taxes.  Call the IRS to resolve this.

[baldietax]

and then for future payments it's easier to pay electronically at irs.gov if you are able, rather than mailing checks etc