Willem van de Velde the Younger, Dutch ships in a calm. Public domain.
Parliament put customs collectors in colonial ports so cargo could not skip English taxes by way of a neighboring colony.
A small planter shipping a few hogsheads met a collector and a bond before the cargo left the river.
A merchant who had been landing Chesapeake tobacco in one colony and sending it onward now faced a duty at the first dock.
London merchants who already paid English customs wanted the collectors there, because rivals had been routing around them.
Planters and merchants had been shipping listed colonial goods from one colony to another and then toward Europe, which skipped the English customs.
The act of 1673 laid a duty, collected in the colonial port, on those goods when they moved between colonies.
It also planted customs officers in the ports, so collection was no longer something that happened only in London.
Plantation Duty Act, 25 Charles II c. 7 (1673)