still, grain was by far the largest export commodity of the commonwealth. the owner of a folwark usually signed a contract with merchants of gdańsk, who controlled 80% of this inland trade, to ship the grain north to that seaport on the baltic sea. many rivers in the commonwealth were used for shipping purposes: the vistula, pilica, bug, san, nida, wieprz, neman. the rivers had relatively developed infrastructure, with river ports and granaries. most of the river shipping moved north, southward transport being less profitable, and barges and rafts were often sold off in gdańsk for lumber. hrodna become an important site after formation of a customs post at augustów in 1569, which became a checkpoint for merchants travelling to the crown lands from the grand duchy.