A system for determining postage rates, said system comprising a lever mounted to pivot about a pivot axis, a wheel disposed at one end of the lever and in contact, at rest, with a conveyor surface on which a mail item is conveyed, measurement means disposed at the other end of the lever for the purpose of measuring the angular displacement of said lever while the mail item is being conveyed and for the purpose of delivering a signal representative of the amplitude of said displacement, and a processor unit connected to the measurement means for determining the thickness of the mail item, the measurement means comprising an encoder provided with a plurality of openings, each of which corresponds to a determined threshold for a postage rate, which openings are distributed over a circular arc over an annular portion and are suitable for going past a stationary sensor comprising an emitter for emitting a light ray and a receiver for receiving said light ray, the processor unit being suitable for counting the number of openings through which said light ray passes as said wheel passes over said mail item, and for deducing from said number the postage rate corresponding to said mail item.