Unfortunately, the USPS tracking code shows only that you sent something to IRS, not that you sent a check.  Was this a payment sent with a tax return?  And now it is clear that they have the tax return, since you have received a collection notice?  Was it a 1040?  Did you mail it to an IRS Service Center, or to the lockbox address in the instructions?  At this point, pay the tax and ask in a letter that the penalty and interest be abated, giving your side of the story.  A copy of the Post Office receipt is helpful, but not conclusive.