2.6 Form Singular Possessives by Adding 's to the Singular Form of the NounThe rules holds true regardless of how the word ends: thus, witness's, Jones's, Congress's, and testatrix's. There are three exceptions to the rule. First, the word its is possessive, it's being the contraction for it is. Second, your and hers, which are absolute possessives, take no apostrophe. Third, biblical and classical names that end with a -zes, or -eez sound take only the apostrophe. Thus,Jesus' Moses' Aristophanes' Socrates'If the possessive form seems awkward to you, rephrase: the laws of Moses instead of Moses' laws, the action of Congress or the congressional action instead of Congress's action.
Given the treatment of Customs as a singular entity, I take it from Garner (my go-to grammar guy) that the CAFC was correct as a matter of formal grammar. Customs's will be my preferred usage henceforth and I am a little embarrassed that I had floated toward treating Customs as a plural noun, which would justify my use of the naked apostrophe. But, that still leaves open the question of Customs's own institutional preference and whether there is any reason for it.
The surprisingly popular practice of omitting the final "s" in all s-ending words is both technically improper and completely illogical. Indeed, the use of an additional "s" accurately reflects proper pronunciation. Whereas an 's produces a clear sound, a mere apostrophe produces no sound at all. Accordingly, if one were to pronounce the sentence, "Kansas' statute is constitutional," it would sound exactly the same as the sentence, "Kansas statute is constitutional." That wouldn't make any sense. Furthermore, it is hard to imagine that law clerks for Justice Thomas go around saying to people, "Hello, I'm Justice Thomas clerk." (Of course, the same analysis applies to people like Jesus and Moses, but they are apparently entitled to some type of "grandfather" exception.)
Given the diversity of opinion, I am feeling empowered to return to my prior practice of dropping the trailing s. I will, though, continue to use a lower case letter c when using customs as an adjective rather than as a proper noun. Thus "customs classification" refers to the practice of classifying merchandise under the tariff schedule. On the other hand, "Customs' classification" refers to the classification Customs assigned to the merchandise.