The Collaborative International Dictionary of English v.0.48:

Tronage \Tron"age\, n. [From Trone a steelyard.]
   A toll or duty paid for weighing wool; also, the act of
   weighing wool. [Obs.] --Nares.
   [1913 Webster]

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

TRONAGE, Eng. law. A customary duty or toll for weighing wool, so called
because it was weighed by a common trona, or beam. Fleta, lib. 2, c. 12.