The Collaborative International Dictionary of English v.0.48:

customs \customs\ n. pl.
   1. money collected under a tariff; a duty imposed on imported
      goods.

   Syn: customs duty, custom, impost.
        [WordNet 1.5]

   2. the government department administering the collection of
      import tariffs. Used with as singular verb.
      [PJC]

   3. that area within an airport, sea port, or other border
      station where freight or the baggage of travellers is
      checked for dutiable materials or contraband; as, it took
      an hour to get through customs.
      [PJC]

WordNet (r) 3.0 (2006):

customs
    n 1: money collected under a tariff [syn: customs, customs
         duty, custom, impost]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "customs":
   Internal Revenue Service, VAT, ad valorem duty, alcohol tax,
   amusement tax, assessment on default, assessor, capital gains tax,
   capitation, capitation tax, corporation tax, customhouse,
   customs duty, death duty, death tax, doomage, duty, estate duty,
   estate tax, excess profits tax, excise, excise tax, exciseman,
   export tax, farmer, federal tax, gabelle, gift tax, head tax,
   import tax, income tax, inheritance tax, internal revenue tax,
   land tax, liquor tax, local tax, luxury tax, nuisance tax,
   personal property tax, poll, poll tax, property tax,
   property-increment tax, protective tariff, provincial tax,
   publican, rates, revenue tariff, revenuer, sales tax, salt tax,
   school tax, severance tax, specific duty, state tax, tariff,
   tariff duty, tax assessor, tax collector, tax farmer, taxer,
   taxman, telephone tax, use tax, value added tax, window tax