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?

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?