The Collaborative International Dictionary of English v.0.48:

Shipper \Ship"per\, n. [See Ship, n., and cf. Skipper.]
   One who sends goods from one place to another not in the same
   city or town, esp. one who sends goods by water.
   [1913 Webster]

WordNet (r) 3.0 (2006):

shipper
    n 1: someone who ships goods
    2: a company in the business of shipping freight

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "shipper":
   Aquarius, Ganymede, Hebe, bearer, bheesty, busboy, caddie,
   cargo handler, carrier, carrier pigeon, carter, common carrier,
   conveyer, coolie, cupbearer, drayman, express, expressman,
   freighter, gun bearer, hauler, homing pigeon, letter carrier,
   litter-bearer, porter, redcap, shield-bearer, skycap, stevedore,
   stretcher-bearer, the Water Bearer, transporter, truck driver,
   trucker, wagoner, water boy, water carrier



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

SHIPPER. One who ships or puts goods on board of a vessel, to be carried to
another place during her voyage. In general, the shipper is bound to pay for
the hire of the vessel, or the freight of the goods. 1 Bouv. Inst. n. 1030.