The Collaborative International Dictionary of English v.0.48:

Towage \Tow"age\, n. [From Tow, v..cf. F. touage.]
   1. The act of towing.
      [1913 Webster]

   2. The price paid for towing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

towage
    n 1: the act of hauling something (as a vehicle) by means of a
         hitch or rope; "the truck gave him a tow to the garage"
         [syn: tow, towage]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "towage":
   admission, admission fee, anchorage, attraction, brokerage,
   carfare, cellarage, charge, charges, cover charge, demand, dockage,
   draft, drawing, drayage, dues, entrance fee, exaction, exactment,
   extraction, fare, fee, haulage, hauling, heaving, hire,
   license fee, pilotage, portage, pull, pulling, pulling power,
   salvage, scot, scot and lot, shot, storage, toll, towing, traction,
   tractive power, tug-of-war, tugging, wharfage



Bouvier's Law Dictionary, Revised 6th Ed (1856):

TOWAGE, contracts. That which is given for towing ships in rivers. Guidon de
la Mer, ch. 16; Poth. Des Avaries, n. 147; 2 Chit. Com. Law, 16.