Little is known about Customs. It is only mentioned once in Book 1: Keeper of the Lost Cities, when Alvar first arrives at Everglen halfway through dinner. Due to his lateness, he apologizes, explaining how he got held up at customs. Customs is likely between the Elvin and Ogre worlds, as Alvar had been supposedly working with the ogres before going through customs.
Customs are unimportant in the later books, though they may be mentioned upon in future books and may become more important.