customs duty vs impost

customs duty and impost both are nouns.

Word	Noun	Adjective	Verb	Adverb
customs duty	Yes	No	No	No
impost	Yes	No	No	No

As nouns, customs duty and impost are synonyms defined as:

Other synonyms of customs duty include custom, customs.

customs duty (noun)	impost (noun)
money collected under a tariff	the lowest stone in an arch -- from which it springs
	money collected under a tariff