Originally Posted by Section 107

USofA taxes alcohol not based on the purchase cost of the alcohol but on the actual total quantity of alcohol - not on the total quantity of liquid - in the bottle(s). So, for example, lets say you had three one-litre bottles of 40% ABV over the duty-free limit. Tax would be calculated on the 1.2 litres of total actual alcohol (and not on the 3 litres of total liquid). The rate may vary depending upon the type of spirit and I seem to recall origin of the alcohol might be a factor, too. It can get pretty complicated. But in general, the actual amount of tax due for most pax is in the small single dollar amounts and not worth the time of the customs officers to assess and they just tell you move along.

If, however, they suspect the alcohol is not for personal consumption but is being imported to be resold - well, then they frequently use their discretion to make it worth their time to assess the tax.