27 CFR 70.434: Assessments.

When additional or delinquent tax liability on tobacco products, and cigarette papers and tubes is disclosed by an investigation or by an examination of the taxpayer's records, a notice (except where delay may jeopardize collection of the tax, or where the amount is nominal or the result of an evident mathematical error) is forwarded to the taxpayer indicating the basis for, and amount of, the liability and affording the taxpayer an opportunity to show cause, in writing, against assessment.