seeing the bottle of hair gel which he could charge duty on这

句子不完整……
看到那瓶他可以征税的发胶……
如果是被动的话
seeing the bottle of hair gel which he could be charged duty on
则是指买发胶的人,不是被动语气的话指的是收银员或海关人员,on 是用来修饰发胶的
He could charge duty on he bottle of hair gel.
句子本身是这样的顺序的,他可以征收那瓶发胶的税.加了seeing看到而把发胶提前了,于是on就放到了最后