custom or customs

hey,i was wondering the difference between custom and customs.
normally,we use [american customs],but is it ok if we say[american custom],cuz by looking up the dictionary,i found that both words could be used to refer to[custom/customs].

otherwise,when using [customs],is it a singular or plural subjecive?

thanks in advance for the help!

Use the plural when you are speaking about the duties, the office, and the airport facilities. Here is what has to say:

a. ( used with a singular or plural verb ) duties imposed by law on imported or, less commonly, exported goods.
b. ( used with a singular verb ) the government department that collects these duties.
c. ( used with a singular verb ) the section of an airport, station, etc., where baggage is checked for contraband and for goods subject to duty.

thanks for such a quick reply!
and it should be like…[ american customs are…].right?

but when do we use the singular form?

thanks again!

What kind of customs are you talking about, fees for imported goods or traditional cultural habits?

got it,for departments in charge of importing/exporting goods,always keep using [customs] and keep the subjective pluralized

And use the singular verb as the definitions above clearly indicate.