This is an automatic function. Once you select a foreign dock for your
merchant ship to sail to and select the type of cargo she will haul (stone,
wood-the default or food), the ship automatically deducts the requisite
amount of cargo from your storage and once she returns, also automatically,
she will deposit the equal amount of gold into your storage area.
Your only decisions are what cargo and what docks.
If you build a new dock closer to the shipping lane, you can change the home
port as well, but aside from that, it's all automatic.
--
Those are the rules. I don't live by 'em, I just make 'em up.
Thanks
The Viper