This is an old thread but it's still relevant. How is CBP duty on excess liquor calculated?
Following site seems to suggest it is subject to a duty "at 3 percent, plus any Internal Revenue Tax (IRT)"
https://www.cbp.gov/travel/internati...toms-duty-info
Determining Customs Duty
The flat duty rate will apply to articles that are dutiable but that cannot be included in your personal exemption, even if you have not exceeded the exemption. For example, alcoholic beverages. If you return from Europe with $200 worth of purchases, including two liters of liquor, one liter will be duty-free under your returning resident personal allowance/exemption. The other will be dutiable at 3 percent, plus any Internal Revenue Tax (IRT) that is due.
For example
If I bring back 2 liters of scotch, one is exempt. Is the other charged at 3% of purchased price plus $2.85 excise tax (based on $13.50 per proof gallon)?